OLIGOTREMA PSAMMITES. 267 



iiiuer surface of the left side of the mantle, and is sunk in the 

 mantle — a feature characteristic of Molgulida}. 



7. The renal organ is largely developed, is situated on the 

 right side of the gut, and has the structure characteristic of 

 the Molgulidfe. 



8. The oviducts and sperm-ducts are paired, a feature 

 which, though not universal in the Molgulidas, is found in a 

 great many members of the family. 



But in classing Oligotrema in so well known and typical a 

 family of simple Ascidians, its many peculiar features must 

 not be lost sight of. These are — the wide separation of the 

 branchial and atrial orifices ; the great reduction in size of 

 the atrial siphon and the concomitant suppression of the 

 atrial lobes ; the highly differentiated pinnate muscular arms ; 

 the great reduction in the size and extent of the branchial 

 sac ; the suppression of the dorsal lamina and the feeble 

 development of the endostyle and peribranchial grooves ; the 

 relatively great length and diameter of the oesophagus, and 

 the presence of a large oesophageal groove extending nearly 

 to the anterior end of the dorsal side of the branchial sac. 



It may be safely inferred, from what we know of the 

 contents of the stomach of this single specimen, that these 

 remarkable modifications of structure are due to chansred 

 habits of life, and particularly to a change from the mode of 

 nutrition characteristic of other Ascidians. The large 

 Amphipod contained in the stomach, Avhether I am right in 

 referring it to the genus Platyscelus or not, clearly belongs 

 to a free-living species, and is neither commensal nor parasitic. 

 It is too large, and must have been too active, to have been 

 swept into the mouth by the action of ciliary currents ; and 

 the same may be said of the relatively large Copepoda, of 

 which abundant fragments are contained in the stomach. Ou 

 the other hand, the size and position of the six tentacular 

 arms, and the powerful and complex musculature with which 

 they are provided, are satisfactory evidence of their eflicieucy 

 as organs for capturing prey. Oligotrema psammites, 

 then, is an Ascidian which captures and feeds on active 



