'98 



ZOOLOGY 



SECT. 



already mentioned, only in the Tetrabranchiata : but in both the 

 Dibranchiata and the Tetrabranchiata certain sensory processess 

 or depressions conjectured to possess an olfactory function are 

 developed on the head. Statocysts are universally present. 



All the Dibranchiata have two nephridia similar in character 

 to those of Sepia, and communicating with one another ; in Octopus 

 they are completely united. In the Tetrabranchiata there are 

 four nephridia, each opening on the exterior. 



The sexes are distinct in all the Cephalopoda, and in addition 

 to the hectocotylised arm, there are frequently other external 

 differences between male and female. In all the Dibranchiata 

 the arrangement of the gonads and gonoducts is, as regards 

 general features, similar to what we find in Sepia. In Octopus, 

 however, there are two oviducts instead of one, and in one other 

 member of the Octopoda (JEledone moschata') the same holds good 

 of the spermiducts. 



Development.- -The development of the Dibranchiata alone is 

 known. The eggs are very large, containing a relatively large 

 amount of food-yolk. They are usually laid in masses or strings 

 embedded in a soft gelatinous, or a tougher, more leathery 

 substance, usually attached to some foreign body ; in some cases 

 each egg, enclosed in a gelatinous sheath, has a longer or shorter 

 stalk. A chorion or delicate transparent egg-membrane, in which 

 there is an aperture the micropyh immediately invests the 

 egg itself. In shape the egg is oval or spherical. The greater 

 part of the comparatively small quantity of protoplasm lies as 



a disc-like elevation on the sur- 



dors face of the yolk on the side of 



the egg at which the micropyle 

 is situated. Continuous with 

 this germinal disc is a thin 

 layer of peripheral protoplasm 

 investing the entire ovum. 



Segmentation (Figs. 697 and 

 698) is incomplete, being con- 

 fined to the germinal disc. At 

 an early stage in the process of 

 division, the blastoderm exhibits 

 a distinct bilateral symmetry. 

 This meroblastic segmentation 

 results in the formation of a 

 nearly circular blastoderm, the 

 outer cells of which tend to 

 separate off. At first the blas- 

 toderm consists of only a single layer of cells the ectoderm, 

 which gradually extends. At a later stage a second layer 

 (Fig. 699, B,C) appears below the margin of the blastoderm, and 



ant 



post 



vent 



'-'7. Segmenting ovum of XiOligO. 

 (l-Yom Knrselielt and Heidcr, after Watase.) 



