THE TUNICATES. THE MANTLE. 419 



In the adult Salpa the oval ventral portion of the brain (Fig. 287, m.ce), from 

 which arise numerous pairs of peripheral nerves, and which formed the floor of 

 the anterior part of the medullary plate, probably represents the condensed rem- 

 nants of several thoracic neuromeres. The dorsal portion, p.ce., represents the 

 primitive forebrain, or supracesophageal ganglion. The cavity, or space between 

 them, represents the rudiments of a cerebral ventricle. Before the neural tube is 

 completely closed, the subneural gland, or primitive stomodaeum, opens into this 

 space and thence to the exterior (Fig. 287, C.) 



The new mouth, or haemostoma, arises from the opposite side of the head, 

 in the region of the cephalic naval, or dorsal organ. 



The Mantle. — ^A characteristic feature of the tunicates is the thick, fibroid, 

 translucent secretion of the ectoderm that forms a flexible covering for them, and 

 that takes the place of the ectodermal skeleton of arthropods. In its general 

 appearance and consistency it is not unlike the softer forms of chiten; but it 

 differs from it chemically, consisting of a special substance, tunecine, said to be 

 identical with cellulose, although it is doubtful whether a more careful analysis 

 will bear out this conclusion. It may be regarded, provisionally as some modifi- 

 cation of chiten, or of a closely related substance. 



At an early stage of development, it is invaded by numerous spindle- or star- 

 shaped cells, differing in character and in origin; some arise from the ectoderm, 

 others from the mesoderm, others from the ovary. It is also broken up by the 

 presence of canals and spaces that permit the invasion of blood-vessels and other 

 tissues, and there are occasional deposits in it of calcareous and silicious specules. 

 In one form, Chelyosoma, it consists of "horny plates" that recall those of 

 cirripeds. 



The mantle of tunicates may be regarded as a special modification of the 

 exoskeleton of arthropods, resembling most nearly that of the cirripeds. The 

 cirripeds have a type of exoskeleton not known to occur elsewhere in the arthropods ; 

 it, therefore, has a special interest and significance for us. Unfortunately, I have not 

 had an opportunity to examine at first hand into the details of its minute structure 

 and development. It develops, according to Gruvel, underneath, or inside, the 

 hypodermis of the mantle, by the secretion of concentric or parallel layers of 

 chiten which then become heavily calcified. It is never cast off, and continues 

 to increase in volume during life. 



In the early stages, the chitenous matrix is crowded with regular spaces, each 

 containing a live cell, which however dies with the progress of mineralization. 

 (Gruvel, p. 362.) In Pachylasma (Fig. 288, A), the shell consists of two principal 

 layers, an outer one of small chambers with thick laminated walls secreted by 

 infoldings of the hypodermis, h, and a basal layer derived from the inner surface 

 of the mantle, nt.l. In Balanus (Fig. 288, B), the outer layer contains large spaces 

 or parietal canals, en., and the hypodermis, h, extends inward in the form of 

 spreading or branching plates. The peripheral spaces or parietal canals, en., 

 were formed by ingrowths from the inner layer of the mantle and contain, for a 



