570 MR. C. STEWART ON CERTAIN ORGANS 



In a full-grown specimen of Dorocidaris these branchiae are about half an inch in 

 length, are gracefully curled upwards, and have on their under surface a double row of 

 compound secondary diverticula, which, largest near the base of the branchiae, are 

 gradually reduced in size towards their tips. The walls of these organs are exceedingly 

 delicate, and contain similar spicular plates to those found in the neighbouring peri- 

 toneum. On either side of the base of the branchiae external to the oblique ligaments of 

 the compasses there is a small group of bulla-like elevations of the peritoneum similar to 

 those found on the sides of the gills. 



The size of the jaw-chamber can clearly be increased by the raising of the compasses 

 when the transverse muscles contract, especially if the jaws be protruded. Such move- 

 ments of the parts are frequently going on ; so that if the chamber communicate with the 

 surrounding water, as it probably does near the tips of the teeth, water would then pass 

 in and out of the space, and bathe the interior of the gills. (I have not yet had an 

 opportunity of examining Asthenosoma, but should think it most probable that similar 

 gills are present there to those found in the Cidaridae.) 



If this hypothesis of one of the functions of the compasses, their muscles, and ligaments 

 be correct, some light is thrown on what has long been a puzzle to me, viz. of what use 

 they are in the economy of the regular Echini. The compasses have been said to be 

 used as levers by which the transverse muscles separate the points of the teeth ; and the 

 oblique ligaments have been thought by some to be muscles approximating the points of 

 the teeth. The ligamentous nature of the latter, however, is quite evident both under 

 the microscope and to the naked eye ; and as such they are described by Valentin and 

 Prof. Agassiz. 



There are two reasons why I consider that the compasses have nothing to do with 

 movements of the teeth. First, there are, exclusive of them, powerful muscles by which 

 all necessary movements of the jaws can be accomplished, viz. five pairs of retractors 

 passing obliquely downwards and inwards from the auricles to the tips of the jaws (pyra- 

 mids), which can also separate the teeth, five pairs of protrusors arising between the 

 auricles and inserted at the bases of the jaws, the whole of the contiguous faces of the 

 jaws giving attachment to a muscle which, passing from one to the other, would approx- 

 imate the teeth. Secondly, the mechanical arrangement of the compasses is unfavour- 

 able to their acting as teeth-movers. It seems, then, possible that the function of the 

 compasses is to raise the peritoneum from the base of the jaws ; at the same time this 

 would stretch the oblique ligaments, and raise this membrane from their sides. A.nother 

 function might also be to remove injurious strain from the madreporic canal when the 

 jaws are protruded. 



Although the pedicellariae of the Cidaridae have been described by many observers, 

 I have not seen the following peculiarity of the armed or chambered pedicellariae men- 

 tioned. In this form, when the jaw ends in a terminal fang, the chamber has a second 

 opening in addition to the large triangular tooth-armed orifice near the tip, the fang 

 itself being hollow, and perforated external to and in front of the tip, a place which 

 corresponds exactly with the opening of the groove in the terminal fang of the gemmi- 



