Primitive Type of Compound Aseidian. 



267 



Musculature in the thoracic region consisting of both longi- 

 tudinal and transverse bundles. Oviduct and vas deferens 

 present in mature zooids. No incubatory diverticulum of the 

 cloaca. 



Fie:. 1. 



Archidistoma nggregatum. — A smnll colony, enlarged. 

 Fig. 2. 



1-v 



Archidistoma aggregatum. — Part of another colony, enlarged, showing the 

 partial freedom of the zooids of a clump, and the tendency of the 

 cloacal openings towards a central position. 



Archidistoma aggregatum^ sp. nov. 



Clumps composed of a small but variable number of zooids. 

 Test arenaceous. Tentacles about thirty in number. Pharynx 

 possessing three rows of straight elongated stigmata ; hori- 

 zontal membranes between the rows of stigmata ; no inter= 

 mediate supporting membranes. Ova lai'ge, containing much 

 food-yolk. 



Archidistoma aggregatum is a connecting-link between the 

 true Distomidse [Distoma^ Cystodites^ Distaplia, Oxycorynia^ 



18* 



