i84 AN INTRODUCTION TO ZOOLOGY, 



body-cavity is traversed by transverse partitions which, 

 divide it into a series of chambers. The nervous 

 system consists of a double dorsal ganglion at the 

 head, with connectives passing round the mouth, 

 together with a double nerve-cord placed in the 

 ventral or lower part of the body. Excretory organs 

 are present, which are called nephridia (little kidneys) 

 or segmental organs, and lie in pairs, opening to the 

 surface on successive segments of the body. There 

 is red blood, circulating in a system of longitudinal 

 blood-vessels. The reproductive organs are bi-sexual, 

 and somewhat complicated in structure. The earth- 

 worm has no free larval stage. For a description of 

 the habits of the earthworm, the student should con- 

 sult Darwin's work on the subject. 



The sub-class Gephybea are a group of worms with 

 very curious larvse; Sipunculus is occasionally met 

 with on our , shores. The Leeches, Hibudinea or 

 DiscoPHOEA, have a sucking mouth-disc, with serrated 

 jaws and no appendages. Some of the leeches make 

 a sort of cocoon in which to lay their eggs, composed 

 of albuminous material, which the young afterwards 

 suck in for food. In other forms, the young suck 

 this material directly from the mother, who carries 

 them attached by their suckers. Some kinds are 

 parasitic. 



TheEoTiPERS, or Eotatoeia (wheel-bearing animals), 

 are placed in the group of Worms. The peculiar 

 feature of the Rotifers is the wheel Organ, a disc 

 with revolving cilia, situated at the anterior end, by 



