230 SIDNEY F. HARM KB. 



descending part of the cryptocyst, partially interrupts the 

 distal region of the body-cavity, so that a pair of spaces, 

 which will be referred to as the '' lateral recesses/' are found 

 one on either side of it. Jullien^ has quite correctly appre- 

 ciated the function of these cavities, which, as he points out, 

 contain muscles. I propose to give a more detailed account 

 of these muscles on a future occasion, and will for the 

 present merely state that they are of three kinds : (i) 

 divaricator muscles, which open the operculum ; (ii) occlusor 

 muscles, for closing this structure ; (iii) depressor muscles. 

 These last are inserted into a linear chitinous thickening 

 (fig. 16, e. s.) of the epitheca on each side. These thicken- 

 ings will be termed the " epithecal sclerites." Their form 

 differs in different species. Busk ^ described them as 

 " furcate spicula," although the furcate appearance was no 

 doubt produced by the insertion of the tendon of the de- 

 pressor muscle into the middle of the sclerite, which is often 

 angulated at this point, so that the vertical tendon seen in 

 perspective with the two halves of the sclerite may have the 

 appearance of a Y. 



I think, with Jullien,^ that there can be little doubt about 

 the function of the depressor muscles. They are to be 

 regarded as special modifications of the parietal muscles of 

 other Polyzoa, whose function is, by depressing the upper 

 ectocyst, to exert a pressure on the fluid of the body-cavity, 

 by which the polypide is extruded.* The tendons, as shown 

 by Jullien, pass through the well-known lateral foramina of 

 such forms as Micropora and Thalamoporella ; and 

 Jullien, in his memoir on the Polyzoa of Cape Horn, suggests 

 no less than thirteen generic types for recent and extinct 

 species characterised by differences in these foramina, which 

 he terms the " opesiules." This application of Jullien's prin- 

 ciple would result in assigning different specimens of S. 



' ' Cap. Horn,' p. I. 77. 



' ' Challenger Rep.,' pt, xxx, p. 75. 



3 'Cap. Horn,' pp. I. 80, 81. 



* Cf. Nitsche, 'Zeitschr. f. wiss. Zool.,' xxi, 1871, p. 426. 



