402 UROCHORDA OR TUNIC ATA. 



Test and Skin. 



The whole body is clothed in a thick test or tunic, which 

 can be readily peeled off from the underlying body wall. 

 This tunic contains a carbohydrate allied to, if not identical 

 with, the cellulose of plants, and also some proteid substance. 

 The whole is at first a true cuticle, but cells soon migrate 

 into it, while at one point blood vessels also enter it from 

 the body, and ramify in all directions. The tunic is begun 

 by secretory prolongations of ectoderm cells, and some 

 ectoderm cells pass out into what is secreted. But the tunic 

 also receives important contributions from mesenchyme cells 

 which migrate into it. Some of them probably act as 

 phagocytes in cases of injury or infection. The " Cellulose " 

 or " tunicin " is common throughout the group, and it is in- 

 teresting to find a characteristically vegetable product in the 

 very passive cuticle of these passive animals. The ectoderm 

 which secretes the tunic is a single layer of cells. 



The Muscular System. 



The muscular system forms beneath the epidermis a 

 netted sheath of unstriped fibres, which are very numerous 

 on the right side of the body, and almost absent on the left. 

 Special sphincters surround the apertures. 



The Nervous System. 



The nervous system, which in the larva consists of a 

 spinal cord with a slight anterior cerebral swelling, is repre- 

 sented in the degenerate adult only by a ganglionic mass, 

 which lies between the two apertures, and gives off a few 

 nerves. 



Sensory Structures. 



Sensory structures in the adult are few and unspecialised. 

 In the larva there is a well developed eye and an auditory 

 organ, both in close connection with the brain. These do 

 not persist in the adult. 



Beneath the ganglion in the adult there lies a small (sub-neural) gland 

 from which a ciliated duct opens into the pharynx. According to some, 

 this corresponds morphologically to the pituitary body (see page 436), 

 and so partially to the pre-oral pit oiAmphioxus (q. v.); its physiological 



