INVERTEBRATE ZOOLOGY. 25 



Class II. Gastrotricha Minute aquatic forms about the size of 



large Infusoria, having an independent 

 origin from Trochozoa and modified in a 

 different manner from the above. They 

 are elongated, somewhat flattened, and 

 possess a double band of cilia on the ven- 

 tral side, derived from the ventral stripe 

 of the Trochozoon. Ciliated zones fail, 

 body generally terminated in a fork. 

 i^ Ic}ifhidiu)ii. 



Class III. Endoprocta Aciuatic forms, mostly marine — probably 



to be viewed as Trochozoa. modified by 

 sessile life. The larvae are free .swim- 

 ming, hnt soon become attached by a 

 stalk developed from the apical pole. 

 The prae-oral zone (Trochus) develops 

 into a crown of ciliated tentacles, within 

 which are month and anus, as well as 

 the genital and nephridial openings. The 

 apical ganglion of the larva disappears 

 and is functionally replaced by another, 

 situated between mouth and amis. The 

 group is generally referred to Bryozoa. 

 {[. V. -^ Urnatella, Loxosoina. 



Class IV. Nematodes Long, cylindrical worms without cilia. 



Body cavity present. Sub-cuticula with 

 foTir longitudinal thickenings, dorsal, ven- 

 tral and lateral, between which lie four 

 bands of longitudinal muscles. Proto- 

 nephridia forming two lateral canals en- 

 cased in the lateral sub-ciiticular thick- 

 enings. Sack gonads long and tubular, 

 often contorted. Ventral nervous system 

 an oesophageal ring, from which issue 

 longitudinal nerves, of which two (dor- 

 sal and ventral) lie in the sub-cuticular 

 ridges. Bisexual with a single exception. 

 Development direct. Free-swimming and 



