292 LOWER INVERTEBRATES. 



species is about four inches in length. The groujj of tooth shells first appears 

 in rocks of Devonian awe. 



Sub-Class II. — Gasteropoda. 



This grouj), which contains by far the largest number of forms (about twenty-five 

 thousand species being known), embraces the molluscs commonly known as snails, 

 slugs, sea-slugs, whelks, cowries, limpets, and the like. ■ In all the head is well 

 developed and bears one or two jtairs of tentacles. The body is usually asymmetrical, 

 owing to the presence of a spiral shell, though this is far from being invariably the 

 case. The alimentary tract is straight or doubled on itself, and usually terminates on 

 one side of the body. A heart is always present, except in the problematical form 

 Entoconcha. Res])iration is effected either by gills, by a pulmonary cavity, or by the 

 general surface of the body. The sexes ai'e separate in the majority of the forms, 

 but are combined in the same individual in the Pulmonata. 



The classification here adopted is based on that of Lankester, which, while it varies 

 greatly from that in common use, has the merit of agreeing well with our knowledge 

 of the anatomy and embryology of the group. The basis of Prof. Lankester's pri- 

 mary divisions is found in the symmetry or torsion of the body. 



Super-Order I. — ISOPLEURA. 



Tlie name given to this division means equal-sided, which emphasizes the most im- 

 portant feature of tlieir structure. They retain in the adult the primitive bilateral 

 symmetry. The alimentary canal traverses the entire length of the body, and termi- 

 nates posteriorly in a median vent. Renal organs, gills, circulatory organs^ and 

 genitals, ai-e jiaired and symmetrical. The ]iedal and visceral nerve cords are straight 

 and parallel, extending the length of the body. 



Order L — CHtETODERM.E. 



This group contains but a single genus, Chwtoderma, which was originally placed 

 among the Gephyrean worms. C. nitidulum is a small, worm-like body, with an 



enlarged head at one end, while the cavity of the 



mantle is found at the other. In this small cavity 



are a pair of small gills. The external integument 



is roughened by minute calcareous s})ines, which 



viG. s-is.-cnakHiennanMduhm. give the body a hairy appearance. The foot is 



obsolete, and the lingual ajiparatus is greatly i-e- 



duced, the lingual ribbon being represented by but a single tooth. Nothing is known 



of the embryology. 



Order II. — NEOMENOIDEA. 



Neomenia is a peculiar genus fotmd on the western coast of Sweden. N. carinata 

 reaches the length of nearly nn inch, grayisli in color, with a shade of rosy red at the 

 posterior end of tlie body. The outer surface is covered with minute spines, giving it 

 a velvety appearance. In shape the body somewhat resembles a pea-pod, a dorsal 

 ridge giving rise to the specific name. The mantle is reduced to a small ring around 



