562 The Animal Kingdom 



The Anatomy of Molgula. — Molgula, a member of the class As- 

 cidiacea, is common in the coastal waters along the Atlantic. 



The adult is saclike in shape and is attached by a narrowed portion 

 at the base. The animal is covered by a tough elastic membrane, the 

 tunic. Inside the tunic is a membranous mantle which contains blood 

 vessels and a few muscle fibers. The internal organs are contained 

 within a central atrial cavity which is located within the tunic. There 

 are two openings into the test, an incurrent and an excurrent siphon. 

 Sex products and waste materials also may pass out with the out- 

 going current of water. Below the incurrent siphon is a tubelike mouth 

 which opens into a large pharynx. The pharynx is enlarged into a 

 branchial sac with many pores through which water passes into the atrial 

 cavity. The pores are surrounded by ciliated cells which create a cur- 

 rent. The surrounding tissue is well supplied with blood vessels and 

 respiratory exchange takes place in this region. 



INCURRENT SIPHON 



TUNIC 



ENOOSTYLE 



EXCURRENT SIPHON 

 GANGLION 



PHARYNX 



ANUS 



Fig. 186. — The anatomy of the tunicate, Molgula. A portion of 

 the branchial sac is removed. 



In the midventral wall of the branchial sac is a conspicuous groove, 

 the endostyle, which is lined with cilia and possesses numerous mucous 

 cells. Small animals which enter with the incoming current of water 

 are caught in the mucus of this groove and by action of the cilia move 

 through the pharyngeal region into the digestive tract proper. The endo- 

 style has at times been considered to be homologous with the thyroid 

 gland of the vertebrates. 



