422 



4. Note on a new and primitive type of Compound Ascidian. 

 1>\ Walter Qarstang, M. A., Berkeley Fellow of the Owens College, Manchester. 



eingeg. 15. August 1891. 



During some dredging operations in the neighbourhood of Ply- 

 mouth which I have recently been enabled to carry on by means of a 

 ( .u\ eminent grant given me by the Royal Society Committee, I met 

 with specimens of a new and interesting Compound Ascidian, which 

 forms the subject of the present note. 



The specimens of this Ascidian were found in moderately shallow 

 water (5 to 15 fathoms attached to stones and shells upon which they 

 formed small, iuconspicuous encrusting colonies freely coated with 

 sand-grains. The colonies possess a thin, spreading carpet-like base 

 of test-substance traversed by stolonial tubes from which zooids spring 

 up at irregular intervals. Sometimes the zooids are entirely free, but 

 usually they are united into small clumps consisting of several indivi- 

 duals the tests of which are partially fused together. The zooids pro- 

 ject from the basal carpet of test to a variable extent; as a rule their 

 height is between six and ten millimetres. They posses a dilated and 

 somewhat globular thoracic region and an elongated, semicylin- 

 drical abdominal region, which is always more slender than the 

 thoracic portion. The zooids bear two distinct apertures, the oral and 

 cloacal openings, of which the former is the larger. Each aperture is 

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

 In the larger groups of zooids there is a distinct tendency to an 

 arrangement of the individuals in such a way that the cloacal aper- 

 tures 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 individvial zooids of a clump from one another. The existence of 

 foreign particles between the zooids of the clumps shews clearly that 

 these have been formed by a process of fusion or concrescence. 



In general structure the Ascidiozooids agree with those of the 

 majority of the Distomidae. The body when removed from the test is 

 seen to be divided into two regions, the thorax and abdomen, which 

 are connected by a slender oesophageal stalk. A mature zooid is from 

 3 — 1 millimetres in length. The musculature is well-developed. In 



