64 



MOLLUSCA. 



Gs 



Oc 



abdomen. They open into the mantle cavity, each through the apex 

 of a papilla. The anterior walls of the sacs are pushed inwards by 

 caecal appendages of the venae cavae (branchial arteries), so as to give 

 rise to a number of racemose lobules projecting into each renal sac 

 (fig. 536). The renal sacs, as in other Molluscs, communicate with 

 the body cavity, which in Sepia is largely developed and contains the 

 heart, generative organs, etc., but in the Octopoda is reduced to a 

 , narrow tubular space 



(" water - vascular sys- 

 tem" of Krohn) and 

 only contains the sexual 

 glands. 



An excretory organ 

 very generally present 

 is the ink-sac. It is a 

 piriform sac, whose duct 

 opens to the exterior 

 with the anus, and 

 empties an intensely 

 black fluid, which sur- 

 rounds the body of the 

 animal as in a black 

 cloud, and so protects 

 it from the pursuit of 

 larger marine animals. 



The Cephalopoda are 

 dioecious. Males and 

 females present external 

 sexual differences which 

 principally concern a 

 particular arm. Accord- 

 ing to the discovery of 

 Steenstrup, one of the 

 arms in the male always becomes modified, hectocotylized as it is 

 called, as an intromittent organ. The two sexes of Argonauta differ 

 considerably, inasmuch as the small male has no shell. 



The sexual glands lie freely in the body cavity. Their products 

 are dehisced into the body cavity, from which they are taken up and 

 conveyed to the exterior by special ducts. The ovary is unpaired and 

 racemose, and the oviduct is a double (Octopoda) or unpaired 

 (usually left) duct opening into the mantle cavity ; it receives in its 



FIG. 537. Anatomy of the body of a female Sepia (after 

 C. Grobben). Ov, ovary in its cavity (body cavity) which 

 is laid open ; Od, oviduct ; Oe, opening of the same ; OdD, 

 oviducal gland ; Nd, nida mental gland ; AD, accessory 

 nidamental gland ; A, kidney : U, ureter ; Lk, canal of 

 the body cavity (water canal) ; Kh, branchial heart ; Kha, 

 perioardial gland (appendage of branchial heart) ; K, gills; 

 Af, anus ; Gst, stellate ganglion. 



