423 



the thoracic region it consists of both longitudinal and transverse fib- 

 res united into bundles that form a strong, square-meshed lattice-work; 

 the longitudinal bundles appear to be arranged in six main groups, 

 corresponding to the number of the oral lobes. In the oesophageal 

 and abdominal regions longitudinal bundles are present but transverse 

 muscles are altogether absent. The ganglion is large and spherical, 

 and the subneural gland is well developed. The buccal tentacles are 

 about thirty in number. The pharynx possesses three rows of straight 

 and elongated stigmata , and two moderately broad horizontal mem- 

 branes with perfectly straight edges. 



In young zooids I have been unable to discover any trace of ovi- 

 duct or vas deferens ; but in mature zooids both are present. The ova 

 are large , and undergo their development in the atrial cavity. There 

 is no special oviducal or cloacal diverticulum for their reception. 



The characters of this Ascidian necessitate the definition of a new 

 genus and species of the family Distomidae. 



Archidistoma, gen. nov. 

 Colonies encrusting, consisting of a spreading basal portion from 

 which arise zooids at irre- Fig t 



gular intervals. Zooids 

 either entirely free or 

 partially fused together to 

 form clump-like aggrega- 

 tions. Oral and cloacal 

 apertures distinct, six- KtgA j|m^--x^^^V 



lobed. Musculature in the _ f\ " : - T 



thoracic region consisting L --4 ;■ — "-■;;;-•; \ — — -S> ^-—- - ' 

 of both longitudinal and ^ ^ distoma aggregatum. 



transverse bundles. Ovi- A gmall colonV) enlarged, 



duct and vas deferens 

 present in mature zooids. No incubatory diverticulum of the cloaca. 



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; horizontal membranes bet- 

 ween the rows of stigmata; no intermediate supporting membranes. 

 Ova large, containing much food-yolk. 



Archidistoma aggregatum is a connecting link between the true 

 Distomidae {Distoma, Cystodites, Distaplia, Oxycorynia, Coletta) and 



