TTPE PBOTOOHOBDATA. 619 



external coat or tunic secreted by the ectoderm of the body 

 and composed, so far as its matrix is concerned, of a substance 

 which is closely related in its chemical characters to vege- 

 table cellulose. 



Inasmuch as considerable variation in form occurs in the 

 class, it will be most convenient to describe in detail what 

 may be considered a typical Tunicate, pointing out subse- 

 quently the more important modifications which occur. 



The simple Ascidians are for the most part ovate in form 

 and usually attached at one end to some support ; in the 

 genus Boltenia, however, the surface of attachment is at the 

 extremity of an elongated stalk, which at the other end passes 

 into the pyriform body. The test or tunic which encloses 

 the body presents usually towards the free end of the body 

 two openings, through one of which, the branchial aperture, 

 water passes into the interior of the body and is eventually 

 expelled through the second or atrial aperture. This test is 

 composed of a matrix sometimes almost homogeneous, some- 

 times fibrillar {Cynthia), and of varying consistency, secreted 

 by the ectoderm. It contains scattered through it numerous 

 cells, which have recently been shown to be mesodermal in 

 origin and to migrate into the test after it has reached a cer- 

 tain thickness. They may remain amoeboid in shape or may 

 develop vacuoles, becoming thereby swollen into bladdeflike 

 structures, or may become the seat of pigment-depositions, 

 or, finally, may secrete spicules of carbonate of lime. 



Extending through the test are numerous branching tubes 

 which communicate with the blood-vessels of the body.' 

 They arise as outgrowths of the blood-vessels and push the 

 body-wall before them, being therefore lined externally by 

 ectoderm, beneath which is a layer of connective tissue, and 

 each is separated into two compartments by a longitudinal 

 partition which does not, however, extend into the bulblike 

 enlargement with which each branch ends, the two compart- 

 ments thus communicating in the bulb and allowing the 

 blood to circulate. 



The shape of the body conforms to that of the test, and 

 opposite the branchial and atrial apertures of the latter is 

 drawn out into two tubular processes, the branchial and atrial 



