L-4KE DWELLING TRICLADS
5
Benazzi (loc. cit.) has reported that they are able to interbreed in the
laboratory but this is not our experience with British material. There
can be no doubt that they are good species on morphological, cytological
and ecological grounds. I n the numerous habitats where they occur together the populations remain distinct even to their epizoic fauna
(Reynoldson, 1956a); M. Benazzi (private communication) has recently
agreed to this view of the situation in nature. Melander et al. (1954) reported a new species Polycelis hepta closely related to P . tenuis but
differing from it in the absence of adenodactyls* and in possessing a
haploid chromosome number of 7 compared with 6 in P . tenuis. However, the recent work of Lender and Moigne (1960), Moigne (1962), and
Benazzi (1963a), has shown conclusively that interbreeding takes place
and the occurrence of adenodactyls is not related strictly to chromosome number. Therefore, P . hepta can no longer be regarded as a good
species. How the ecology of the different karyotypes varies is not
known.
Confusion has also been rife over distinctions between Dugesia lugubris
and Dug. polychroa (0. Schmidt). Funaioli (1951) on the basis of morphological examination concluded that they were the same species, with
lugubris as the correct specific name. However, the extensive work of
Benazzi (e.g. 1957) and his school has shown the complexity of the
situation by recording several karyotypes in this species-complex and
he recommended (1963b) that the distinction in Schmidt’s sense (1860)
should not be retained. Dendrocoelum lacteurn has been a less controversial species and identification is straightforward.
Undoubtedly there are several karyotypes in some of these species,
but Dend. lacteum has been recorded only in the diploid condition
(Dahm, 1961). Polycelis nigra occurs mainly in the diploid condition
with 2n = 16 (Melander et al., 1954; Moigne, 1962), but Benazzi (1963a)
has also recorded a triploid variety. Polycelis tenuis is more complex,
possessing two haploid chromosome numbers of 6 and 7 with triploids
built onto both (Lepori, 1955; Moigne, 1962; Benazzi, 1963a). Asummary
of the karyotypes in Dugesia lugubris is given in Benazzi (1959) and
this species seems the most complex of the Planariidae considered here.
All the British material of this species which has been examined except
one population from N. Ireland belongs to either Biotype B or C of
Benazzi, a triplo-hexaploid (Magagnini, 1961 ; M.- Benazzi, private communication). The polyploid races of these three species generally reproduce by pseudogamic development of the oocytes, i.e. sperm from
another individual is necessary to stimulate development of the egg but
no fusion of chromosomes occurs.
* Pear-shaped glandular structures opening into the genital atrium.
5
Benazzi (loc. cit.) has reported that they are able to interbreed in the
laboratory but this is not our experience with British material. There
can be no doubt that they are good species on morphological, cytological
and ecological grounds. I n the numerous habitats where they occur together the populations remain distinct even to their epizoic fauna
(Reynoldson, 1956a); M. Benazzi (private communication) has recently
agreed to this view of the situation in nature. Melander et al. (1954) reported a new species Polycelis hepta closely related to P . tenuis but
differing from it in the absence of adenodactyls* and in possessing a
haploid chromosome number of 7 compared with 6 in P . tenuis. However, the recent work of Lender and Moigne (1960), Moigne (1962), and
Benazzi (1963a), has shown conclusively that interbreeding takes place
and the occurrence of adenodactyls is not related strictly to chromosome number. Therefore, P . hepta can no longer be regarded as a good
species. How the ecology of the different karyotypes varies is not
known.
Confusion has also been rife over distinctions between Dugesia lugubris
and Dug. polychroa (0. Schmidt). Funaioli (1951) on the basis of morphological examination concluded that they were the same species, with
lugubris as the correct specific name. However, the extensive work of
Benazzi (e.g. 1957) and his school has shown the complexity of the
situation by recording several karyotypes in this species-complex and
he recommended (1963b) that the distinction in Schmidt’s sense (1860)
should not be retained. Dendrocoelum lacteurn has been a less controversial species and identification is straightforward.
Undoubtedly there are several karyotypes in some of these species,
but Dend. lacteum has been recorded only in the diploid condition
(Dahm, 1961). Polycelis nigra occurs mainly in the diploid condition
with 2n = 16 (Melander et al., 1954; Moigne, 1962), but Benazzi (1963a)
has also recorded a triploid variety. Polycelis tenuis is more complex,
possessing two haploid chromosome numbers of 6 and 7 with triploids
built onto both (Lepori, 1955; Moigne, 1962; Benazzi, 1963a). Asummary
of the karyotypes in Dugesia lugubris is given in Benazzi (1959) and
this species seems the most complex of the Planariidae considered here.
All the British material of this species which has been examined except
one population from N. Ireland belongs to either Biotype B or C of
Benazzi, a triplo-hexaploid (Magagnini, 1961 ; M.- Benazzi, private communication). The polyploid races of these three species generally reproduce by pseudogamic development of the oocytes, i.e. sperm from
another individual is necessary to stimulate development of the egg but
no fusion of chromosomes occurs.
* Pear-shaped glandular structures opening into the genital atrium.