890 THE ANIMAL CARRIERS OF DISEASES 



A. Gonadial and reno-pericardial cavities commvimca.te-— I sopleura, 



Siphonopoda. 



B. Gonadial and reno-pericardial cavities separate — Prorhipidoglossa- 



morpha. 



The Isopleura contains the ' Chitons', and the Siphonopoda the * Cepha- 

 lopods, with which we are not concerned. 



Grade Prorhipidoglossomorpha Grobben. 

 Definition. — MoUusca in which the gonadial and reno-pericardial cavities 

 are separate, the foot is wholly posterior to the head, and a visceral commissure 

 is present. 



Classification. — The Prorhipidoglossomorpha are divided into three classes 

 as follows: — 



A. Body bilaterally symmetrical : — 



I. Mantle united ventrally to form a tube. No ctenidia — Class I., 

 Scaphopoda Bronn. 

 II. Mantle not so united; ctenidia present — Class II., Lamellibranchia 

 de Blainville. 



B. Body asymmetrical — Class III., Gastropoda Cuvier. 

 We are only concerned with the third class. 



Class Gastropoda Cuvier. 



Definition. — Prorhipidoglossomorpha with asymmetrical organization, with 

 well-developed head, with shell formed in one piece and spirally coiled, at 

 least in the larva. 



Classification. — The class may be divided into two subclasses : — 



A. Visceral commissure twisted into a figure of eight, mostly dioecious — 



Streptoneura. 



B. Visceral commissure not so twisted, with shortened visceral com- 



missure monoecious — Euthyneura. 



Subclass I.: Streptoneura Spengel. 

 Definition. — Gastropoda, dioecious, with a few aberrant genera; maximum 

 torsion of visceral mass and commissure. Head with only one pair of ten- 

 tacles. 



Classification. — The Streptoneura are divided into two orders: — 



A. Nervous system not concentrated. Infraoesophageal commissure 



present. Ctenidia bipectinate and free at their distal ends— 

 Aspidobranchia. 



B. Nervous system somewhat concentrated. Infraoesophageal com- 



missure present. Ctenidium monopectinate and attached to 

 mantle along its whole length — Pectinibranchia. 

 Only the last concerns us. 



Order Pectinibranchia. 

 Definition. — Streptoneura as defined above. 



Classification. — The Pectinibranchia are divided into two suborders: — 



A. Without proboscis, pallial siphon, or Leibleim's unpaired oesophageal 



poison gland — Tcenioglossa. 



B. With proboscis, pallial siphon, and Leibleim's poison gland— S/ewo- 



glossa. 



Only the Taenioglossa are of importance to us. 



Suborder Taenioglossa. 

 Definition. — Pectinibranchia with the characters given above, with three 

 teeth, one lateral and two marginals, on each side of the median tooth of the 

 radula. 



