320 PHYSIOLOGY OF THE INVERTEBRATA. 



rest ; these help to hide it, and the secretion from the glands 

 is therefore of the greatest service. 



It will be noticed that the nervous system of the Echino- 

 dermata is much more highly developed than that of the 

 Cmlcnterata. 



The Triciioscolices. 



According to De Quatrefages the nervous system of the 

 Tiirhellaria consists of two ganglia situated in the anterior 

 end of the body, from which, in addition to other branches, a 

 longitudinal nerve-cord extends backwards on each side of 

 the body. As a general rule, the lateral trunks exhibit 

 ganglionic masses, and from these ganglia nerves are given 

 off. These " may beciome approximated on the ventral side 

 of the body, thereby showing a tendency to the formation 

 of the double ganglionated chain characteristic of higher 

 worms." 



In the Rotifcra, the nervous system consists of a large 

 ganglion situated on one side of the body near the trochal 

 disc. This ganglion, sometimes divided into two portions, 

 gives off nervous filaments. 



The nervous system of the Ccstoidca consists of two 

 longitudinal lateral nerve-trunks, which run down the body 

 externally to the main canals of the excretory system. In 

 the so-called head of the animal, where they are swollen 

 (ganglia), they are united by a transverse commissure. 



Dr. G. Joseph* has recently examined the nervous system 

 of the Ccstoidca. The results arrived at are — (a) That the 

 two cerebral ganglia are in many cases (Ta'nia transvcrsalis, 

 T. ropludocera) connected, not by a single dorsal commissure, 

 but by two, separated by a matrix and muscle-processes ; (b) 

 that each cerebral ganglion is triple, consisting of a median 

 and two smaller (dorsal and ventral) ganglia separated by 

 muscle-processes, as is best seen in Tccnia crassicollis ; (c) that 

 in the bladder-worm, before evagination of the hooks, the 



* BiohgiscJies C'entralUatt, vol. 6, p. 733. 



