lx 



THE VOYAGE OF H.M.S. CHALLENGER. 



mal margin of the velar muscle. Its parallel and thickly compacted circular fibres 

 sometimes run as uninterrupted rings on the whole subumbrella, or are sometimes divided 

 by four, eight, sixteen or more radial septa into an equal number of separate plates. The 

 coronal muscle is divided into four interradial plates in a large number of Craspedotse; into 

 eight adradial, e.g., in the Cubomedusse (PL XXVI.), into eight principal (four perradial 

 and four interradial) plates in Pericolpa (fig. F), into sixteen subradial in the Pectyllidse 

 (Pis. IV., VIII.) ; in Periphylki, on the other hand, eight of the coronal plates are principal, 

 and eight adradial (PI. XIX.). Very often (namely, in the larger Acraspedse) the sup- 

 porting plate rises below the coronal muscle in the form of simpler or more complex cir- 

 cular folds, so that the muscular system lying above it commands a more extensive 



Fig. F. Pericolpa qitadrigata (Peromedusre, Pericolpidre). 



Subunibral aspect, (oi) Sense clubs (interradial). (oa) Ampulla at their bases. (() Tentacles (perradial). 

 (bl) Lobe pouches, (bit) Horseshoe-shaped canals of the pouches. (M) Peronia between the two 

 limbs of the canal, (s) Genitalia, (mk) Root-muscles of the tentacles. (md l ) Perradial deltoid 

 muscles, (md?) Interradial deltoid muscles, (aft) Oral ribs, (ar) Oral grooves on their inner sur- 

 face, (ai) Oral tavniola. (tr) Tentacle roots, (be) Coronal pouches, (mc) Coronal muscles. 



surface of insertions in a limited space (Pis. XIX. -XXII. , mc). In most Craspedotae, the 

 coronal muscle is comparatively broader, and covers the larger part of the subumbrella as 

 connected plates, whilst in most Acraspedse it is narrower and more limited to their peri- 

 phery ; it is often forcibly thrust back by the deltoid muscles, which extend themselves 

 at its expense (fig. F, md). The velar muscle (" musculus velaris ") forms the third 

 distal section of the subunibral circular muscular system. In the Craspedotas it repre- 

 sents the most important element of the velum (Pis. IV.-XIV.) and is separated by the 

 ventral nerve ring from the contiguous distal margin of the coronal muscle. It is repre- 

 sented in the Acraspedse, by the analogous (but not homologous) coronal muscle of the 



