142 THE VOYAGE OF H.M.S. 'CHALLENGER. 



termination of a side branch from the ancestral Botryllidaj which has undergone 

 considerable modification (see table, p. 150). 



Four generic groups are recognised amongst existing Botryllidse. Of these, two 

 (Sarcobotrylloides and Polycydus) form thick massive colonies, while the other two 

 {Botryllus and Botrylloides) form thin incrusting colonies. In Polycydus and 

 Botryllus, again, the systems are regular and stellate (Fig. 26) ; while in Botrylloides 

 and Sarcohotrylloides the systems form irregular linear or lirauched arrangements. 

 The Ascidiozooids vary in structure according to the shape of the systems. They 

 are short and ovate, with the atrial aperture fer from the anterior end of the body, 

 where the systems are regular and stellate ; while they are longer and more cylindrical, 

 with the atrial aperture placed close to the branchial, in tlie forms with irregular or 

 linear systems. 



Probably these two opposite conditions of the systems and the Ascidiozooids charac- 

 terised two branches into which the ancestral Botryllidse divided (see table, p. 150). 

 Both series had the colony of moderate thickness ; but the one, that leading to Botryllus 

 and Polycydus, had the systems regular and the Ascidiozooids ovate ; while the other, 

 that leading to Botrylloides and Sarcohotrylloides, had the systems elongated and the 

 Ascidiozooids cylindrical. Probably the first of these conditions was that which 

 obtained amongst the more primitive Botryllidse (those occupying the Hue N. before its 

 division), since we find at the present day in young colonies of Botrylloides the systems 

 often commencing in stellate forms like those of Botryllus, and then gradually 

 elongating and branching to form the more complicated arrangements characteristic 

 of Botrylloides. 



The difference in the structure of the Ascidiozooids can readily be seen to be 

 merely the result of the shape of the systems. When the system is stellate, each 

 Ascidiozooid forms one of the rays of the star (see Fig. 26, p. 141), with the branchial 

 aperture placed at the anterior end, the point farthest from the centre ; and as the 

 atrial apertures open into the common cloacal cavity in the centre of the system, 

 they are necessarily placed far from the anterior end of the body. But when the 

 systems become greatly elongated and branched, it is no longer possible for each 

 atrial aperture to reach the common cloacal cavity, and consequently that cavity 

 becomes prolonged into a series of canals, which penetrate all parts of the system 

 running between the Ascidiozooids. As a result of this, it Ijeing no longer necessary 

 for the Ascidiozooid to be stretched out in order to reach the common cloaca, the body 

 comes to be more vertically placed, and the atrial aperture returns to its ancestral 

 position near the anterior end. 



Each branch of the Botryllidse afterwards divided into two groups (see table, 

 p. 150), in the first of which the colony became thinner and more of an incrusting 

 film {Botryllus on the one hand, and Botrylloides on the other), while in the second 



