CHORDATES IN GENERAL 



151 



in most of the common forms. Some are brilliantly tinted with color. 

 The adult is covered externally by a cellulose coat or tunic (test), 

 which is secreted by the cells of the underlying mantle. Inside the 

 mantle is the extensive atrial cavity. On the dorsal (unattached) 

 side of the body are two funnellike siphons. The anterior one is the 

 branchial siphon (oral funnel or incurrent siphon or mouth) and the 

 other is the atrial siphon (atrial funnel, excurrent siphon, or atrio- 

 pore). When the tunic of Molgula is removed, one may see most of 

 the internal organs through the transparent mantle. 



These animals are hermaphroditic or monoecious. Each has two 

 compound sets of gonads, one on the left side in the loop of the 

 intestine and the other on the right side of the body. Some of the 



Incurrent siphon 



Excurrent siphon 



II Mantle 



Tunic 



r¥\ Qonqlion 



-Anus 

 -^^ Genital duct 



^'%--- Ovary 

 T^\~~ 0\qestive glands 

 " — tsophaqus 



— -\ntest\m 



- -^stomach 

 — branchial fold 



- - Endostyfe 



- /Atrium 



- - Phar^r)K 



Fig. 48. — Diagram of Molgula manhattensis from the left side to show the struc- 

 ture with the courses of water and food indicated by arrows. 



sessile tunicates, as Molgula, reproduce by budding. The life history 

 of the tunicate is one of interest. ^Cross-fertilization is the rule; 

 that is, spermatozoa from one individual usually fertilize ova from 

 another; however, there may be exceptions to this. The fertilization 

 occurs in the water outside the body. The eggs hatch to produce 

 larvae somewhat similar to amphibian tadpoles which are free-swim- 

 ming. The larva possesses the typical notochord, gills, and nerve 

 cord of Chordata. For some reason it then settles on the bottom 

 and attaches itself by adhesive papillae located in the anteroventral 

 position. Some authors express it by saying this larva settles on its 



