266 Mr. W. Garstang on a New and 



by six well-marked lobes of triangular or semicircular shape. 

 In the larger groups of zooicls there is a distinct tendency to 

 an arrangement of the individuals in such a way that the 

 cloacal apertures are situated towards the centre of each 

 clump, the oral apertures towards the periphery. 



The test for the most part is covered with sand-grains, 

 whereby the colonies are rendered highly inconspicuous. 

 The adhesion of sand-grains is of interest in considering the 

 process by which the clumps are formed. In the majority of 

 the clumps examined, the sand-grains form a complete sheath 

 around each zooid ; they not only adhere to the test of the 

 zooids upon their external faces, but they also separate the 

 individual zooids of a clump from one another. The existence 

 of foreign particles between the zooids of the clumps shows 

 clearly that these have been formed by a process of fusion or 

 concrescence. 



In general structure the ascidiozooids agree with tliose of 

 the majority of the Distomidse. The body, when removed 

 from tiie test, is seen to be divided into two regions, the 

 thorax and abdomen, which are connected by a slender oeso- 

 phageal stalk. A mature zooid is from 3 to 4 millim. in 

 length. The musculature is well developed. In the thoracic 

 region it consists of both longitudinal and transverse fibres 

 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 

 membranes with perfectly straight edges. 



In young zooids I have been unable to discover any trace 

 of oviduct 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 Distomidse : — 



Archidistoma, gen. nov. 



Colonies incrusting ; consisting of a spreading basal portion 



from which arise zooids at irregular intervals. Zooids either 



entirely free or partially fused together to form clump-like 



aggregations. Oral and cloacal apertures distinct, six-lobed. 



