REPORT ON THE TUNICATA. 145 



thus rendering the colony thick and massive. In none of the Polystyelidse have the 

 Ascidiozooids come to be arranged in systems, and no common cloacal cavities have l)een 

 formed, — the atrial apertures of all the Ascidiozooids open independently on the exterior 

 of the colony. In this respect the colony is in the same stage of differentiation as that 

 reached by most Distomida^ while the other groups of Compound Ascidians {e.g. 

 Botryllidse, Polyclinidse, Didemnidse) have advanced a stage farther by the formation 

 of systems with common cloacal cavities (see Fig. 26, p. 141). 



It is possible that the family Botryllidse may be more closely allied to the 

 Polj^styelida) than I have shov/n in the table. The line N., in place of springing from 

 the primitive Cynthiidse, may possibly have been derived from the Polj-styelidse, near 

 the point where Goodsiria and Synstyela diverged. In that case the evolution of the 

 primitive Botryllidte would consist in the gradual formation of systems in the colony, 

 and the complete disappearance of all traces of folds in the branchial sac. 



The primitive Cyntliiidse at the point M. (table, p. 150), after the separation 

 of Bathyoncus and the ancestral Styelinse, must have acquired compound or branched 

 tentacles, as they were the common ancestors of the Cynthinse, the Bolteninse, and the 

 Molgulidfe. At the same time, the folds in the branchial sac became more marked and 

 increased in number. At or aljout the point 0. in the table the important line of 

 descent leading to the family ]\Iolgulidje probably diverged, while the main branch 

 was continued onwards to form the Cynthinse. 



In the ancestral JMolgulidse the branchial sac became still more complicated by the 

 curving of the stigmata and the fine interstigmatic vessels so as to produce a series of 

 more or less perfect spirals. The folds in the branchial sac remained of large size, and 

 the compound tentacles became greatly branched. At the same time the test liecame 

 prolonged into a number of branched hair-like processes, containing blood-vessels, and 

 probably corresponding to the stolons of the Clavelinidse. These processes have the 

 power of taking up and attaching sand grains to their surfaces so as to form a sandy 

 investment all over the body. 



The remarkable genus Evgyra was derived from these ancestral Molgulidse (see 

 table, p. 150), and has undergone modification and a slight degeneration. The stigmata 

 have become more completely spirally coiled than in any other Molgulid, but the 

 folds in the branchial sac have completely disappeared, while the internal longitudinal 

 bars have been widened and flattened to form ribbon-like bands. 



Amongst the species of the more t}^)ical Molgulidse^ a considerable amount of 

 differentiation has taken place. In some forms (e.g. Ascopera, and some species of 

 Molgula) the sandy investment and the hair-like processes of the test have been lost, 

 while in others the body has become pedunculated. In some cases (e.g. Molgula 

 carpenteri) the stigmata are not curved, and closely resemble those of the family 



^ See H. de Lacaze-Duthiers, Arch. d. Zool. exper., torn, iii., 1874, and torn, vi., 1879. 

 (ZOOL. CHAIX. EXP. — PART LXXVL — 1888.) Gggg 19 



