454 THE ANIMAL KINGDOM. 



The sexes are united in Pteropoda^ euthyneurous Gastropoda Aniso- 

 pleura, and a few Lamellibranchiata. Among the last-named different 

 parts of the same sexual gland may be male or female ; or, as in Ostrea 

 edulis and the other groups referred to, there is an hermaphrodite gland 

 in which sperm and ova develope near to one another, or side by side. 

 The glands themselves are simple in structure. In Cephalopoda they 

 are developed in the walls of the pericardium, as they appear to be in the 

 Gastropodan Neomeniae and Chaetoderma. In these latter the genital 

 products are set free through the nephridia, in Cephalopoda by special 

 ducts. The glands in other Mollusca are continuous with their own 

 ducts, but in some Lamellibranchiata the latter may open into a common 

 sinus with the nephridial ducts, or as in Scaphopoda into the nephridium. 

 There is no accessory genital apparatus of any kind in Lamellibranchiata 

 and Scaphopoda. But other Mollusca possess at least an intromittent 

 organ, in Cephalopoda a modified arm, in others a process of the 

 body-walls, either remote from or connected to the genital aperture ; 

 a gland which secretes albumen for the ova ; and in some cases organs 

 such as a uterine dilatation, receptaculum seminis, vesicula seminalis, 

 together with special glands. These accessory organs are most com- 

 plicated in Pteropoda and hermaphrodite Gastropoda. Impregnation of 

 the ovum may take place externally to the organism, as in Cephalopoda 

 (? all), Scaphopoda^ Chiton and Zygobranchia among Gastropoda, and 

 most Lamellibranchiata ; within the genital duct in Pteropoda and most 

 Gastropoda. The spermatozoa are contained in spermatophores in Cephalo- 

 poda and pulmonate Gastropoda. The ova vary much in the amount of 

 food-yolk they contain, and therefore in their mode of segmentation. 

 They are largest in Cephalopoda where the only part of the ovum that 

 segments is, as in the Sauropsida, a disc of protoplasm. The ovum is 

 devoid of primary egg-membranes in Gastropoda and Cephalopoda. 



The Mollusca with the exception of Cephalopoda pass through a 

 typical larval development, in two stages a Trochosphere and a Veliger 

 stage. The Molluscan Trochosphere has a ventral mouth, bent intestine, 

 and ventral or terminal anus. It has a praeoral lobe, variable in size, 

 but encircled by a band of cilia the velum at its base, with in some 

 cases a postoral band of fine cilia (Mc'Murrich, A. N. H. (5), xvi. 1885). 

 It is especially characterised by a ventral projection between the mouth 

 and anus, the rudimentary foot, and by a dorsally placed depression 

 lined by epiblast the shell-gland. It passes into the Veliger stage in 

 which the velum enlarges, and in the larvae of Pteropoda and Gastropoda 

 is drawn out into at least two lobes, one right, the other left : the shell 

 gland may secrete a chitinous plug, but it flattens out (except in a few 



kidney' in certain regions of the integument and walls of the pericardium of some Gastropoda 

 Opisthobranchia\ Trinchese, Arch. Biol. Ital. iv. 1884. 



