86 



TUNICATA. 



The integument is sometimes gelatinous and sometimes of leathery 

 or cartilaginous consistency, and is often clear as crystal or trans- 

 parent, but sometimes opaque and variously coloured. The outer 

 surface is smooth or warty, sometimes spiny or felted. This external 



integument, which completely 

 envelopes the body, is called the 

 external mantle (tunica), and was 

 formerly regarded as a sort of 

 shell and compared to the bivalve 

 shell of the Lamellibranchs. This 

 view seemed to be supported 

 by the interesting discovery of 

 Lacaze-Duthiers* that there are 

 Ascidians in which the stiff 

 cartilaginous tunic is split into 

 two separate valves which can 

 be closed by special muscles 

 (Chevreulius]. As a matter of 

 fact, this is simply an external 

 analogy, for the mantle space 

 corresponds to an atrial cavity, 

 and the branchial sac to the 

 pharyngeal sac. The substance 

 of the mantle arises as a cuticu- 

 lar excretion ; it consists of a 

 matrix containing cellulose and 

 cells, and therefore with respect 

 to its structure is a kind of 

 connective tissue. In the colonial 

 Tunicates the external mantles 

 of all the individuals may fuse 



Pi a .5S8.-<?te V dKnaZ^adi/or m M(rfegneaniinal), together to form a Common 



somewhat diagrammatic. 0, Mouth ; Br, gills > mags. 



End, endostyle ; Oe, oesophagus; G, nervous ^ , , . . 



centre; MB, stomach; A't, cioacai spac; A, Beneath the saccular mantle 



exhalent pore ; Af, anus; GD, genital gland; Ji es the body wall of the animal, 

 Qg, genital duct ; Sf, stolons. ... . 



the outer cellular layer of which 



is applied to the mantle and represents the ectodermal epithelium 

 which has produced the mantle and also the subjacent so-called 

 internal mantle layer. Within the latter all the organs of the 



* Lacaze-Duthiers, " Sur un nouveau d'Ascidien." Ann. des Sc. Nat. V. 

 Ser., Tom. IV., 1865. 



