472 THE ANIMAL KINGDOM. 



into the pericardial cavity, and through it communicate with the nephridia. 

 Neomenia carinata has been said to possess lateral male ducts with cal- 

 careous penes. 



In Chiton the ova are impregnated in the pallial groove. Segmenta- 

 tion is total, and at first equal. There is a Veliger with a flagellum in the 

 centre of the velar area, a pair of lateral eyes in the mantle furrow, and a 

 foot-gland which aborts. The nerve-cords are formed from the ectoderm. 

 Transverse dorsal furrows make their appearance, in which the cuticula 

 thickens, and the thickenings are the rudiments of the shell-plates. There 

 does not appear to be a shell-gland. 



The sub-class Isopkura is divisible into three orders: (i) Polyplacophora 

 (Chiton and its allies) ; (2) Neomeniae, including the genera Neomenia and Proneo- 

 menia and (3) Chaetoderma, with a single genus of the same name. 



Polyplacophora. Chitons of Adriatic, Haller, Arb. Zool. Inst. Wien, iv. 1883; 

 v. 1884. Haddon, Challenger Reports, xv. 1886. Eyes and sensory organs, 

 Moseley, Q. J. M. xxv. 1885. Nephridium, &c., Sedgwick, P. R. S. xxxiii. 1883 ; 

 Van Bemmelen, Z. A. vi. 1883; cf. Haller, M. J. xi. p. 41. Eggs and envelopes, 

 Sabatier, Revue Sc. Nat. Paris and Montpellier (3), iv. Development, Kowalesky, 

 Annales Mus. Nat. Hist. Marseilles, i. 1883. 



For other literature and that relative to Neomeniae and Chaetoderma, see 

 Hubrecht on Amphineura, Q. J. M. xxii. 1882. This author is stated to be pre- 

 paring a monograph of the group in the ' Fauna and Flora of the Gulf of Naples.' 



SUB-CLASS 2. GASTROPODA ANISOPLEURA. 



Gastropoda, in which the primitive bilateral symmetry is retained in 

 the head and foot, whilst the visceral dome with the mantle fold is twisted 

 from behind round the right side more or less to the front, so that the 

 ctenidia, amis, nephridial and generative pores are placed either on the right 

 side of the body or more or less anteriorly, the primitive right ctenidium thus 

 appearing on the left side of the anus or body. The visceral dome frequently 

 continues to grow with a similar but spiral twist, and its surface, with the 

 mantle, is generally protected by a more or less spiral shell. The primitive 

 left ctenidium and left (? right) nephridium x usually atrophy. The genital 

 gland and duct are single. A superficial secondary bilateral symmetry is 

 sometimes acquired (Natantia, Opisthobranchia). The foot has commonly 

 a pedal gland. 



The foot is generally broad and flat, and sometimes much expanded. 

 It is divided into a distinct pro- and meso-podium, and a metapodium in 

 many Reptant Azygobranchia and all Natantia. The pro- and meso- 



1 So says Professor Lankester ; but recent researches make it probable that the primitive right 

 and not the left, nephridium atrophies. See p. 479 and note. The word 'primitive' implies the 

 organs of the right and left sides before the occurrence of any spiral twist. 



