138 LILIAN SHELDON. 



Lacaze-Duthiers; 1 and Herdman 2 gives a short account of some 

 forms found by the Challenger. 



External Characters. 



The individuals are small, varying in length from half to 

 little more than an eighth of an inch, and each is almost as 

 broad as it is long. It is wide and flattened at its base, by 

 which it is attached to the rock, there being no stalk or 

 peduncle. A great number of individuals live upon the same 

 piece of rock, and are very closely applied to one another by 

 their sides, so as to form a compact mass ; but any one can be 

 easily separated from the rest. The lateral pressure to which 

 they are thus subjected causes the individuals to vary much in 

 shape. 



In colour they are a bright brownish red. They are quite 

 opaque, so that it is not possible to make out any of the inter- 

 nal structure without removing the test. 



The oral and atrial apertures are four-lobed. 



The Test. 



The test, which gives the red colour to the animal, is fairly 

 thin, and in life is so closely applied to the body wall that it 

 is difficult to remove it without tearing the epidermis. After 

 preservation it can be removed with comparative ease, although 

 it still adheres to the body wall. Except at its base, where 

 fragments of the rock on which it lived generally remain 

 attached to it, the test is free from sand, calcareous spicules, 

 or other foreign matter. 



As seen in sections it is composed of a homogeneous matrix 

 with a few scattered cells. It also possesses a complicated 

 system of vessels, which are lined by short columnar cells. In 

 picrocarmine-glycerine preparations of the test these vessels 

 are clearly seen, as well as numerous pigment cells, which are 



1 Lacaze-Duthiers, Henri de, " Hist,oire des Ascidies Simples des Cotes de 

 France," ' Archives de Zool. Exp. et Gen.,' tome vi, 1877. 



3 Herdman, W. A., 'Challenger Report on the Tuuicata,' Part I, "Ascidise 

 simplices." 



