208 COMPARATIVE ANATOMY. 



connected pieces ; in them the only separate muscles on the shell 

 are those which move the spicules or spicular processes ; those too 

 which are found in the interior of the body serve only to move 

 definite organs, as for example the muscles of the masticatory appa- 

 ratus of the Sea-Urchin. In the Spatangida, however, the shell is 

 movable at one point. 



Very different relations obtain in the Holothuro'i'da, where the 

 absence of large skeletal pieces leads to a proportionate development 

 of the muscular system. Its connection with the integument is very 

 definite. There is a circular layer of muscle beneath the connective 

 tissue of the skin ; within this are five longitudinal bands of muscle, 

 which are sometimes divided (Fig. 113, m) and are separated by 

 intermediate spaces of varying breadth ; these bands are inserted 

 anteriorly into the already described calcareous ring (R). They are 

 connected through the five pieces which are pierced so as to give 

 passage to the nerves and ambulacral vessels. The circular layer is 

 continuous only in the Synapta3 ; in the Holothuriaj it is broken 

 through at the radii, so that it really consists of interradial transverse 

 fibrous tracts only. 



Nervous System. 

 § 169. 



The principal parts of the nervous system of the Echinoderma 

 consist of a number of trunks, corresponding with the antimeres of 

 the body; they are placed ventrally and have a radial direction, and 

 are also connected by commissures around the oesophagus. These 

 commissures are formed by each of the nerve-trunks, which ac- 

 companies the ambulacral vessels, being divided into two halves 

 near the mouth ; these pass to each side, and become connected 

 with the corresponding halves of the neighbouring nerve-trunks, 

 which go to meet them. In this way a ring is formed, which sur- 

 rounds the gullet, which, however, on account of its mode of 

 formation, must not be compared with the oesophageal ring of the 

 Vermes. Each of the radial trunks corresponds rather to the 

 ventral ganglionic chain, or ventral medulla of the Annulata ; and 

 the commissures therefore between several such trunks are connec- 

 tions between the ventral medulla 1 , which owe their origin to the 

 concrescence of several incompletely separated persons. 



As to their more special characters ; the position of the radial 

 nerves immediately below the well-developed epithelial layer of the 

 ambulacral groove (Fig. 100, A n) in the Astero'ida and Comatula? is 

 an important point ; for it indicates that the relations between the 

 nervous system and the ectoderm are almost direct. Perhaps this 

 position is due to the mode of development of the radial nerves, and 

 possibly we here meet with a very low condition, in which differentia- 

 tion is not completed. The fact that processes of the epithelial 



