162 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Pteropoda. 
Morphology, Classification, and Chorology of Pteropoda.* — Dr. 
J. E. V. Boas, from a close comparison of the thecosomatoiis and gymno- 
somatous divisions of Pteropods, comes to the conclusion that they ought 
to be regarded as two independent groups. They are both so close to 
the Opisthobranchiata that they may be regarded as suborders of that 
order of Gastropods ; as the present names of Thecosomata and Gymno- 
somata are inconvenient, they may be respectively replaced by Euptero- 
poda and Pterota. As the groups are distinct, they must not be together 
compared with the Cephalopoda, but independently of one another ; if 
the Thecosomata are compared with Cephalopods, they are seen to have 
no other point of comparison except the pallial cavity; and, if we 
consider the isolated position which the Cephalopoda hold in relation to 
the rest of the Mollusca, it becomes clear that this single point of re- 
semblance cannot be regarded as an argument in favour of a closer 
connection between the two groups. The same considerations apply to 
the Gymnosomata — in a single point, the presence of suekers on their 
arms, they resemble the Cephalopoda. The author regards these 
resemblances as analogical only. 
After discussing in detail the organization of the Thecosomata, Dr. 
Boas considers what place they occupy among the Opisthobranchiata. 
He thinks it obvious that they approach most closely to the Tecti- 
branchiata. In a large number of these latter, as in the Thecosomata, 
there is a muscular gizzard, which is provided with a varying number of 
teeth or horny plates. In the Bullidae, as in the Thecosomata, the liver 
is a compact organ, which opens behind the gizzard, and lias two ex- 
cretory ducts in the former and one in the latter. The genital organs, 
and especially the penis, are also very characteristic ; in both the genital 
orifice is simple, the penis is a sac which can be evaginated, and only 
communicates with the genital orifice by a groove. 
In the Thecosomata the cerebral ganglia are connected with one 
another by a long commissure, while all the other commissures are very 
short, so that all the large ganglia are placed near one another ; of such 
Opisthobranchs as have had the anatomy of their nervous system 
adequately studied, the Bullinm most resemble the Thecosomata. The 
existence of a well-developed shell and of an operculum in Limacina are 
further proofs of the author’s position, for it is only in Tornatella (one of 
the Bullidse) that there is an operculum. The Thecosomata are divided 
into the Limacinidse, Hyaleidae, and Cyrabuliidse, and their genera and 
species are described. The last-named family is the one ^\hich bears 
the strongest marks of adaptation to a pelagic mode of life — the “ shell'’ 
is semi-gelatinous, the viscera are concentrated into a nucleus, the nuclei 
are reduced, the pigment is limited to the nucleus, and the rest of the 
body is transparent. 
The Gymnosomata have their chief affinities also with the Tecti- 
branchiata, as is shown by the characters of their generative apparatus ; 
in other points there is not so close an agreement, but there is nothing to 
render it improbable. The characters which speak in its favour are the 
presence of a gill on the right side in some genera and the large number 
* Skrift. K. Danske Vid. Selsk., i. (1886) 231 pp. (8 pis.)- 
