270 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Social and Compound Ascidians.* — Wm. E. Kitter has a note on 

 Maurice Caullery's recent formation of a new genus — Synclavella — to 

 contain a species of Clavelina in which the ascidiozooids are not merely 

 connected by a stolon, but have also a common test. Hitter opposes the 

 new genus on the ground that in Perophora annectens, on the coast of 

 California, both the social and compound conditions occur, sometimes 

 on the same rock or even in the same colony. Of the two conditions the 

 social seems the commoner, but transitional forms are frequent. Kitter 

 first described this remarkable phenomenon seven years ago. 



Protostigmata of Molgula manhattensis De Kay.f — Dr. Arthur 

 Willey finds that the succession of stages known in the nepionic period 

 of Ascidiidre also occurs in at least the above-mentioned representative 

 of the Molgulidas. He describes the distigmatic, tetrastigmatic, penta- 

 stigmatic, and hexastigmatic stages, and compares them with those of 

 Ciona intestinalis. The first two stigmata (A and B) produce by abstric- 

 tion (C and D) ; a fifth (E) arises by independent perforation ; the 

 sixth (F) is abstricted from the fifth. 



INVERTEBRATA. 

 Mollusca. 

 7. Gastropoda. 



Structure of Pleurotomaria beyrichii.J — M. F. Woodward has 

 had an opportunity of examining three specimens of this interesting 

 Mollusc, and compares his results with those obtained by other authors 

 for the other species. Generally, he finds that Pleurotomaria is a typical 

 example of a zygobranchiate Diotocardian. As in P. quoyana, there are 

 no sharply marked specialised regions in the radula, so that the members 

 of the genus are to be regarded as primitive members of the Bhipi- 

 doglossa. In the reduction of the right gill an approach is made to 

 the azygobranchiate Diotocardia. As regards the nervous system, the 

 ganglionic cells are uniformly distributed through the connectives, the 

 commissures, and even the large nerves, and there are therefore no dis- 

 tinct ganglia — an extremely primitive condition. The visceral loop 

 originates, roughly speaking, half-way between the cerebral and pedal 

 regions, and in this respect Pleurotomaria approaches the archi-taenio- 

 glossate Paludina and Nassopsis. The author finds no special concen- 

 tration of ganglionic cells just above the position which would be 

 occupied by the pedal ganglion, as described by Bouvier and Fischer, 

 but regards a slight concentration of ganglion cells at the point of origin 

 of the visceral loop as indicating the position at which the pleural 

 ganglion would occur if it were differentiated. He believes that the 

 position of the gill skeleton, and the possession of a special stomach- 

 cajcum, indicate an ancestry common to that of the Cephalopoda, and 

 that Pleurotomaria may be regarded as a form very closely related to 

 the stock from which the Monotocardia originated. 



Cleavage in Trochus.§ — A. Eobert has studied the segmentation of 

 the ova of Trochus magus and Tr. conuloides. It is very similar in the 



» Amer. Nat., xxxv. (1901) pp. 230-1. Cf. this Journal, 1900, p. 453. 



t Quart. Journ. Micr. Sci., xliv. (1900) pp. 141-60 (1 pi.). 



X Tom. cit., pp. 215-68 (4 pis.). 



§ Comptes Reudus, cxxxii. (1901) pp. 995-7. 



