CLAVELINA. 149 



transparency when preserved in spirit, and especially 

 as C. lepadiformis is large enough for successful dis- 

 section, considerable information may be acquired from 

 specimens in this state. 



The test (PI. XLIX, fig. 1) is smooth, perfectly 

 hyaline, thin, semi-cartilaginous, and elastic; it adheres 

 to the mantle, but can be removed with ease. 



The mantle is diaphanous, very delicate, and mem- 

 branous ; it is not very muscular, the fibres being only 

 feebly developed. A few longitudinal, widely-separated 

 fibres are observed in the thoracic portion ; but the 

 longitudinal contractions would seem to be mainly 

 dependent on two rather stout bundles of fibres which 

 pass from the thorax to near the lower extremity of 

 the abdomen, in the wall of which their extremities are 

 inserted. 



In this form the thorax and abdomen are distinct. 

 In the former the branchial sac is suspended ; in the 

 latter the digestive and reproductive organs are 

 lodged. 



The branchial sac (PI. L, fig. 1) nearly fills the 

 thoracic portion, and extends the full length of the 

 cavity, leaving on each side the atrial spaces, and the 

 cloaca in front. The aerating surface is a simple, rect- 

 angular network, in which the meshes are narrower and 

 longer than usual (PL XLIX, fig. 2) ; they are formed 

 by secondary vessels extending vertically between the 

 large transverse or primary channels, of which there 

 are fourteen or fifteen of about equal size, placed at 

 regular intervals. The walls of those vessels give 

 support to as many ribbon-like membranes, which 

 reach from the endostyle to the oral filaments, and are 

 arranged along the ventral margin occupying the place 

 of the usual oral lamina. The free margins of those 

 ribbon-like membranes project into the branchial cavity 

 and appear to be bounded by a small vessel, the surface 

 of which seems to be ciliated. These membranes are 

 undoubtedly the homologues of the transverse mem- 

 branes in connexion with the branchial papillae in 



