44* 



THE ANIMAL KINGDOM. 



Larvacea the muscles are striated (Fol), and are restricted to the tail, where 

 they form two bands, which are segmented into myomeres. The connective 

 tissue cells are often pigmented. The caudal notochord of the Larvacea 

 and of the larva is rod-like, and composed of a clear substance, like 

 cartilage in consistency, inclosed within a delicate sheath, beneath which 

 are remains of the cells of which the tail was originally composed. In the 

 larva the clear substance appears first of all as a series of discs one behind 

 the other, which ultimately fuse. 



The nervous system in the larva consists of two anterior dilatations, 

 the foremost vesicular, the second with a fine canal, and of a posterior 

 nerve, which runs along the tail. At the root of the tail there is a 

 ganglionic enlargement, from which a pair of nerves passes off to the 

 corresponding myomere. Similar pairs of nerves are given off to succeeding 

 myomeres. Their points of origin are dilated, but apparently not gan- 

 glionic in all instances. The anterior dilatation of the brain opens ex- 

 ternally by a pore, left when the neural groove closes to form the neural 

 tube. In Larvacea the nervous system is somewhat similar. There is an 

 anterior pyriform ganglion, connected by a nerve containing an internal 

 fine canal to a ganglion at the root of the tail. The caudal nerve gives off 

 successive sets of nerves, and lies on the left side of the tail, owing to a 

 twist in that organ. In other Urochorda the nervous system is either from 

 the first a simple ganglion, or becomes reduced to it during the meta- 

 morphosis. This ganglion gives off branching nerves, both anteriorly and 

 posteriorly 1 . A process of the ganglion in Larvacea and Thaliacea extends 

 towards a ciliated tubular depression opening dorsally and anteriorly into 

 the pharynx, in front of the peripharyngeal bands. The cavity of the 

 ganglion has been said to open into this tube in the larva and the young 

 Salpa. The ciliated depression ( = ciliated sac, dorsal or olfactory tubercle) 

 alluded to is found universally in Urochorda ; and in many Ascidiae Sim- 

 plices and A. Compositae a system of glandular tubes, surrounded by blood 

 sinuses, underlies the nerve ganglion, and opens into the depression. This 

 neural gland has been considered to be renal in function, and to be the 

 homologue of the pituitary body of Vertebrata. As to organs of special 

 sense, the oral tentacles of Ascidiidae, &c., are probably tactile in function. 

 So too certain modified ectoderm cells bearing cilia, surrounding the oral 

 aperture of Larvacea, and fringing even the margins and tip of the tail in 

 Kowalewskaia. They occur also on the lobes of the oral and atrial aper- 

 tures in Doliohim and Salpa, as well as on the body surface. Orange-red 



1 In certain simple Ascidians the ganglion is connected posteriorly to a nerve, 'the visceral 

 ganglionic cord,' which contains ganglion cells, and passes along the dorsal edge of the pharynx, 

 then between the rectum and so-called oesophagus, inclines to the right, and ends between the two 

 lobes of the liver. It is derived from the main cord of the larva, which extends from the brain to 

 the root of the tail and along that organ. See E. van Beneden and Julin, Archives de Biol, v. 1884, 

 PP- S^-S 21 * and cap. II. p. 337 et seqq. 



