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4. Stylocheilus longicauda (Fig. 14). Satis- 
factory results were obtained from two speci- 
mens. Both had 17 bivalents during diakinesis. 
5. Haminoea linda (Fig. 16). We were able 
to obtain cells that were satisfactory for study 
from only one specimen. These cells had 17 
bivalents during Metaphase I. 
6. Haminoea musetta (Fig. 15). The two 
specimens studied both had 17 bivalents in cells 
at the diakinesis stage. The diakinesis bivalents 
of one such cell are shown in Figure 15. 
7. Lathophthalmus smaragdinus (Fig. 17). 
Two specimens had meiotic cells that were satis- 
factory for chromosome number determinations. 
All dividing cells from which accurate counts 
could be made had 18 bivalents. 
8. Smaragdinella calyculata (Fig. 18). Satis- 
factory results were obtained from two speci- 
mens. Both had 18 bivalents during diakinesis 
and Metaphase I. 
9. Onchidella evelinae (Figs. 19 and 20). 
The two individuals on which we were able to 
obtain satisfactory observations both had 18 
bivalents during diakinesis and Metaphase I. 
DISCUSSION 
The chromosome numbers of the eight genera 
of opisthobranchiate mollusks presented here 
add to the information previously obtained by 
reliable authors. Of the species studied five 
belong to three families not studied in past re- 
ports, i.e., the Favorinidae (Nudibranchia) , and 
the Atyidae and Smaragdinellidae (Cephala- 
spidea) . 
Dendrodoris nigra (Nudibranchia, Dendro- 
dorididae) was studied previously by Inaba and 
Hirota (1958). We found the same number 
of chromosomes (n=13) for this species from 
Eniwetok as they reported it to have from 
Japan. Herviella mietta (Nudibranchia, Favor- 
inidae) from Eniwetok also had a haploid 
number of 13, which adds to the growing 
information regarding the extreme conservative- 
ness of chromosome numbers of most opistho- 
branchs. All 16 species of nudibranchs (belong- 
ing to nine different families) studied so far 
have this same haploid number, n=13. 
Among the Cephalaspidea three species have 
been studied previously (Inaba, 1959^ and per- 
sonal communication), each belonging to a dif- 
ferent family. Each of these three species had 
a haploid number of 17. In the present in- 
vestigation two species from each of two addi- 
tional families were studied. Haminoea linda 
and H. musetta (Bullacea, Atyidae) had a hap- 
loid chromosome number of 17, but Smarag- 
dinella calyculata and Lathophthalmus smarag- 
dinus (Philinacea, Smaragdinellidae) each had 
the haploid number 18. It will be interesting to 
see if species of the other families of the Phili- 
nacea (the Philinidae, Scaphandridae, Agla- 
jidae, Gastropteridae, and Runcinidae) also 
have 18 pairs of chromosomes. If so, this would 
separate this superfamily from all other cepha- 
laspideids and, additionally, from all other cyto- 
logically known entomotaenids and anaspideids 
and from most of the sacoglossans. The haploid 
number 18 in this group may have another 
significance in that it seems to strengthen Pel- 
seneer’s (1893) and Boettger’s (1955) views 
regarding the origin of the Basommatophora 
from the Cephalaspidea, since the haploid num- 
ber 18 is basic for the Basommatophora. 
Dolahrifera dolahrifera and Stylocheilus 
longicauda (Anaspidea, Aplysiidae) both had 
the haploid number 17, as did the two species 
studied from this family by Inaba (1959^). 
Onchidella evelinae (Soleolifera, Onchidii- 
dae) had a haploid number of 18, which is 
one bivalent more than that reported by Inaba 
(1961^) for O. kurodai of the same genus from 
Japan, but n=18 is the same number reported 
by Natarajan (1959) for Onchidium verracu- 
latum from India. Much more cytological 
information is desirable for the various species i 
belonging to this aberrant order, which is some- I 
times placed with the "pulmonates” (e.g., 
Baker, 1955). 
The conservativeness of chromosome number 
in the opisthobranchs indicates that these mol- 
lusks are extremely resistant to changes in 
chromosome numbers, regardless of major 
evolved morphological diversities within the 1 
various groups, and that certain major divisions 
(i.e., the Nudibranchia and the orders with 
n=17 and higher chromosome numbers) have 
probably been separated for an extremely long 
geological time. In this regard, Bosellia mimet- 
ica is either an extremely aberrant species, or i 
its cytological evolution has been much more 
rapid than has been the evolution of its gross 
