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TEGUMENTARY ORGANS 





one genus we have a test resembling cartilage ; in another, like bone ; 

 in a third, like connective tissue. It may either be without vessels 

 or traversed by branched and ramified vascular processes of the body. 

 It is in all cases, however, a product of the metamorphosis of the 

 ecderon of the outer tunic or mantle and, complicated as its structure 



may be, corresponds morphologically 

 with the shell of other molluscs, or 

 with the epidermis of the higher 

 animals. In fact, if a section be 

 made through the outer tunic and 

 test of an Ascidian, as mfig. 314. A, 

 taking care not to disturb the natural 

 relations of the parts, we observe at 

 the line of contact between the outer 

 tunic and the test {a) an arrangement 

 of the parts very closely resembling 

 what exists at the junction of the 

 derma with the rete Malpighii in the 

 human skin. The outer tunic, like 

 the former, is constituted by bundles 

 of rudimentary connective tissue 

 which run inwards to form sheaths 

 around the muscles, leaving between 

 them spaces, the sinuses of the blood 

 vascular system, while externally 

 they fuse together into a homo- 

 . geneous substance containing endo- 

 plasts, which is thrown into processes 

 and passes insensibly outwards into 

 a layer of similar substance, with 

 very close-set endoplasts almost 

 perpendicular to its surface, which 

 forms the commencement of the . 

 proper test. Externally to this rete 

 Malpighii, the deposit of cellulose 

 commences ; the tissue undergoing 

 at the same time a fibrous metamorphosis. The line a is therefore 

 a protomorphic line, and the test is the product of the growth and con- 

 version, by deposit of cellulose within its elements, of a true ecderon. 



The separation of the ecderon (or test) from the enderon (outer 

 tunic) takes place with great readiness in some Ascidians, as Phallusia, 

 &c. ; while in others, such as Boltenia, many Cynthise, Salpse, &c., it 



Fig. 314. 



