36 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Mollusca. 



Latent Segmentation in Molluscs.* — Werner Marchand finds hints 

 of latent metamerism in the four gills of Nautilus, in the four gonads 

 of some bivalves (such as Poromya), in the ccelom pouches, and so on, 

 and infers that the ancestral molluscs had at least three segments — a 

 head segment and two gonadial segments, with separate ducts. He 

 favours Gunther's suggestion that Chgetognatha are related to the 

 ancestral stock from which Molluscs arose, and concludes by maintaining 

 (what his paper at least can hardly be said to warrant) that " we have 

 every reason for speaking of a latent segmentation in molluscs." 



a. Cephalopoda. 



Hectocotylisation and Luminosity in Cuttlefishes. f — W. E. 

 Hoyle, in his Presidential Address to the Zoological Section of the 

 British Association, discusses some questions suggested by the study of 

 Cephalopods. Attention is first directed to hectocotylisation, and a 

 useful list of genera is given showing the position of the hectocotylised 

 arm or arms, where this peculiar modification occurs. In this connec- 

 tion he discusses the systematic value of this character, for in every 

 family (with one exception, Sepiolidse) the position of the hectocotylised 

 arm is constant within the limits of the family. The position of 

 Spirula forms the next subject of inquiry. It is regarded as the repre- 

 sentative of a distinct family, and it is not unlikely that it may one day 

 become the type of a division co-equal with Myopsida and (Egopsida. 

 The genera Idiosepius, Sepiadarium, and Sepioloidea are then discussed. 

 It is concluded that the position of the hectocotylised arm is not by 

 itself a sufficient guide to the systematic position of doubtful forms. 



After discussing Jaeckel's view that the Orthoceras type was firmly 

 attached, and that Belemnites were anchored in the mud, the author 

 proceeds to the luminous organs. These have now been observed in 

 29 out of about 70 genera of Decapods, and have been found to present 

 a most interesting variety in position and structure. A valuable list is 

 given of the luminous Cephalopods, with bibliographical references, and 

 with notes on the position of the organ, which may occur in nine 

 different situations. It may be noted that the luminous organs are 

 practically confined to the ventral surface of the animal. Another 

 remarkable fact is the existence of organs concealed beneath the mantle 

 and beneath the integument covering the eyeball, which can only be 

 effective by reason of the transparency of the tissues in the living 

 creature. The organs may be glandular or non-glandular, and the latter 

 may be simple, without special optical apparatus, or complex, with 

 more or fewer of the following structures : pigment layer, reflector, lens, 

 and diaphragm. These organs occur in so many and such scattered 

 families that their origin must be polyphyletic. Even in the same 

 species they are not all on the same plan. It is plausible to suppose 

 that they serve as recognition marks, and that they act as searchlights 

 playing over the ground. The production of the light is a phenomenon 



* Biol. Centralbl., xxvii. (1907) pp. 721-8. 

 f Rep. Brit. Assoc, 1907, 20 pp. 



