TUNICATA. 91 



Ilolgula gigantea, which is one of the largest of the Ascidians, 

 and which attains a length of over thirteen inches, forms a tough 

 conical sac ; the branchial and atrial orifices at the npper end 

 have six and four lobes respectively. The test is leathery, smooth 

 above, but coated with sand below. The exhibited specimen, 

 which comes from the Straits of Magellan, has several specimens 

 of the stalked Boltenia Ugumen attached to the lower part of the 

 test. 



The cmious Molgula oculata (Fig. 10) has a soft oval or rounded 

 body coated with sand. The branchial and atrial orifices have res- 

 pectively six and four lobes. Specimens grow attached to the rocks 

 and also live free in the sand. The surface of the test is provided 

 with hairs, which adhere to the rocks and collect particles of sand. 

 The adhesion not being very firm, specimens are easily detached by 



Fig. 10. 



^4- 





Molgula oculata. 

 a, Lrancliial ; b, atrial orifice. 



currents and collected into heaps by the eddies ; when living in the 

 sand only the two dark orifices are visible. The sand coating has 

 been supposed to confer protection by mimicry of the environment ; 

 but Professor Lacaze Duthiers found, much to his chagrin, that 

 the sandy tests of his specimens were of no avail in securing them 

 from being devoured by crabs who seemed to scent their prey 

 from afar. 



In the family Gynthiidce, the test is usually leathery, the 

 branchial and atrial apertures four-lobed, and the branchial sac 

 folded into longitudinal pleats. 



