PHYLUM TUNICATA (UKOCHORDA). 



cuticular layer containing cellulose. It is lined internally 

 by a layer of ectoderm cells which in the first instance 

 secretes it. The remainder of the body wall, consisting of un- 

 striped muscle and connective tissue and in the region of the 

 atrial cavity of lining atrial epithelium, constitutes the mantle. 

 In the living animal it is in contact with the ectoderm, but in 

 spirit specimens it frequently shrinks away leaving a space be- 

 tween the two, the only points of contact being the mouth and 

 atrial apertures, and the point near the hind end of the endostyle, 

 where the blood-vessels pass across from the mantle to the test. 



The test is a 

 cuticular secre- 

 tion of the ecto- 

 derm, of very 

 various consist- 

 ency and colour 

 in the different 

 forms. When it 

 is first formed it 

 is structureless. 

 but it frequently 

 becomes fibril - 

 lated and in 

 most Tunicates 

 processes of the 

 body wall con- 

 taining blood- 

 vessels and nu- 

 cleated proto- 

 plasmic elements 

 soon make their 

 way into it. The 

 vascular proces- 

 ses may arise at 



one or more points. They branch in the test and eventually end in 

 terminal swellings. Each blood-vessel is double, except in the terminal 

 bulb, where the two are in communication (Fig. 2, A). The nucleated 

 protoplasmic elements of the test are for the most part mesodermal 

 in origin, having passed through the ectoderm. Some of them have 

 a peculiar vesicular form, having developed large vacuoles (vesicular 

 cells). Others contain pigment, while yet others may secrete calcareous 

 (Synascidians) or siliceous (Salps) spicules. 



The mantle contains a considerable development of muscular 

 fibres, which confer upon it a great power of contractility. 

 At the apertures the muscles are arranged as sphincters. 



The mouth is surrounded by eight small lobe-like projections of 

 the mantle, between which red pigment spots are placed. It leads 



FlG. 2. Vessel from the test of Phallusia mammillata x 250 (after 

 Seeliger from Bronn). A end of test-vessel in surface view. B 

 series of four sections (l-iv) through a terminal test- vessel made at 

 the lines marked l-iv in A. The arrows indicate the direction of 

 the blood-stream, bb cavity of blood-vessel ; bz blood corpuscles ; 

 ec ectodermal wall of the vessel ; g gelatinous septum, which 

 continues the separation between the two blood-streams for a short 

 distance (n and in) into the terminal bulb. 



