THE BRAXCHIOPODA. 243 



median ridge, on each side of which a curious concentric mark- 

 ing, indicating the position of the shell-gland (Fig. 63, J5, x), 

 is visible. This gland is a coiled tube with clear contents, 

 which, according to Claus, opens on the base of the first pair 

 of thoracic appendages, immediately behind the second max- 

 illae. Where the free joins the fixed portion of the carapace, 

 the ridge is abruptly terminated by a transverse depression. 

 A little distance in front of this is another deeper transverse 

 groove, close to which, in the middle line, are the two reni- 

 form compound eyes, converging toward one another ante- 

 riorly (Fig. 63, B, i'). 



The ventral surface of the anterior division of the carapace 

 (Fig. 63, G) presents a flattened, semilunar, subfrontal area, 

 as in Limulus, behind which it slopes upward on all sides 

 into the posterior division, thus forming a wide chamber, in 

 which the anterior thoracico-abdominal segments are lodged. 

 In the middle line, the subfrontal plate sends back a long and 

 wide process, movably articulated with it, and rounded at its 

 free end — the labrum ; above and behind which the mouth 

 and gnathites are situated. Behind these follow twenty-six 

 spinulose thoracico-abdominal segments ; the anterior twenty 

 of which bear the swimming-feet, while the twenty-sixth, 

 much larger than the others, is produced into an incurved 

 point posteriorly, and carries the anus and the terminal setae. 



The compound eyes, as has been said, are seated upon the 

 upper surface of the anterior division of the carapace. On the 

 under surface, just above and behind the posterior boundary 

 of the subfrontal area, and on each side of the labrum (Fig. 63, 

 C, lb), is a delicate jointed filament — the antennule (Fig. 63, 

 (7, n'). Behind this Zaddach found, in some specimens of 

 Apus cancriformis, a second very small filament, the rudiment 

 of the antenna, which in the larva is so large and important 

 an organ ; but I have observed nothing of the kind in A. gla- 

 cialls. On each side of the labrum is a large, convex, strong, 

 toothed mandible, and the aperture of the mouth is bounded 

 posteriorly by a profoundly divided plate, the metastoma. 

 Succeeding this are two pairs of small maxillae, the second 

 pair being foliaceous, and almost rudimentary. Behind these 

 appendages, a cervical fold marks off the boundary between 

 the head and the thorax, and at the same time corresponds 

 with the commencement of the free portion of the carapace. 

 Whether the carapace is also to a certain extent attached to 

 the first thoracic somite, as Grube states, 1 or whether it is en- 



1 u 



Bemerkungen iiber die Phyllopoden," p. 81. 



