292 LOWER INVERTEBRATES. 



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

 in rocks of Devonian age. 



SuB-CiiAss n. — Gasteropoda. 



This group, 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 pairs 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 tei-minates on 

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

 Mitoconcha. Respiration is effected eithei- by gills, by a pulmonary cavity, or by the 

 general surface of the body. The sexes are 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 

 gi-eatly 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. 



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

 portant feature of their 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, are paired and symmetrical. The pedal and visceral nerve cords are straight 

 and parallel, extending the length of the body. 



Order I. — CH^TODERM^. 



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

 among the Gephyrean worms. C. nitidnlum 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 spines, which 

 ¥10. 325.- chatodermanitididum. ^ give the body a hairy appearance. The foot is 



obsolete, and the lingual apparatus is gi-eatly re- 

 duced, the lingual ribbon being represented by but a single tooth. Xothing is known 

 of the embryology. 



Order II. — NEOMENOIDEA. 



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

 reaches the length of nearly an inch, grayish in color, with a shade of ros}- red at the 

 posterior end of the 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 



