Characteristics of Cephalopoda. xci 



products into the cavity of a compartment of the perivisceral 

 space, whence they are taken up by the open mouth of the effe- 

 rent generative duct, as the ova in most Vertebrata are taken up 

 by the Fallopian tubes. The oviducts are bilaterally symmetrical 

 in the sub-order Odopoda, and in the genus Ommastrephes ; but 

 both male and female efferent ducts are in all other Cephalopoda 

 unpaired like the glands with which they are in relation. The 

 male and female Cephalopoda are distinguishable from each other 

 by external differences, and most markedly by the modification of 

 one of the arms or tentacles of the males to serve as an intro- 

 mittent organ, the so-called ' Hectocotylus,' which in some species, 

 Argonauta argo, Octopus carena, Tremoctojms violaceiis, and Tremoc- 

 tojms Quoi/amis, is set free from the male animal, and, probably, 

 reproduced after each act of sexual congress. The Cephalopoda 

 differ from other Invertebrata in the very large proportion of the 

 yolk which escapes segmentation ; and with the large size of the 

 nutritive yolk we may correlate the fact that the embryos do not 

 undergo any metamorphosis after leaving the Qgg. 



The Tetrabranchiate differ from the Dibranchiate Cephalopoda in 

 the following particidars besides those which their name connotes. 

 They have an external shell, to which the body is attached by 

 strong muscles, but no ink-bag; their tentacles are much more 

 numerous, but are not armed with the suckers which gave the 

 D'lhrancJiiata their name of ' Acetabulifera;' their internal cartila- 

 ginous skeleton is limited to the head, and does not there form a 

 perfect ring; the two halves of their 'funnel' are not anchylosed, 

 but project by two free edges into the mantle cavity, where they 

 form a tube by mutual apposition ; their blood- vascular system ap- 

 pears to be less sharply differentiated from their water- vascular or 

 perivisceral, and their eyes are pedmiculate. The families Naiitil'ulae 

 and Ammonitidae make up the entire order Tetrahranchiafa, and 

 are represented in the Silurian formations by numerous genera, 

 species, and individuals, whilst at the present time the order is 

 represented only by the rare Nautili, the living species of which 

 are variously stated to be two, four, or six. The more highly- 

 organized DibrancJdata have attained their greatest development as 

 an order in the modern Period, but make their first appearance in 

 the Triassic formation. They are divided into three sub-orders — 

 the Becapoda calc'iphora, to which the existing SpiruUdae and 



