298 THE ANATOMY OF INVERTEBRATED ANIMALS. 



ophthalmic sterna, as they bend back from the sternal flex- 

 ure. This rostral septum, therefore, abuts below and behind 

 on the epistoma, and constitutes a sort of partition (Fig. 76, 

 a), by which the cavities in which the antennules and eyes of 

 the two sides are lodged are completely separated from one 

 another. The lateral portions of the rostrum form a flattened 

 roof over the inner portions of these cavities, which contain 

 the bases of the ophthalmic peduncles and the antennules ; 

 but the outer angles of the rostrum are produced downward 

 (d), to form the supraciliary lobe. The outer half of the 

 lateral cavities or chambers is more excavated, and is bound- 

 ed by a strong pointed process, the external orbitar lobe, 

 which is divisible into a supraorbital and suborbital portion. 

 The latter passes gradually into a strong process of the sub- 

 hepatic region, called the internal suborbitar lobe (Fig. 76, 

 e) ; this turns forward and upward toward the supraciliary 

 lobe, which it approaches, but does not meet, the base of the 

 antenna being, as it were, wedged between the two. 



The supraciliary, external orbitar, and internal suborbi- 

 tar lobes, and the antennae, thus together circumscribe a 

 cavity widely open in front, which is called the orbit, inas- 

 much as it lodges the terminal portion of the ophthalmic 

 peduncles, with the eyes which they support. The proximal 

 portions of the peduncles pass through the comparatively 

 narrow opening by which the inner and outer chambers com- 

 municate, between the antenna and the supraciliary process, 

 and are inserted as usual into the articular cavities on each 

 side of the ophthalmic sternum, which is narrow, and hardly 

 wider than the septum. It thus comes to pass that the eyes, 

 lodged in their orbits, appear to be altogether external to 

 the antennules, the enlarged bases of which hide the oph- 

 thalmic peduncles, and appear to be the sole contents of the 

 inner division of the subfrontal chamber ; but the true posi- 

 tion of the eyes is precisely the same as in Astacus, that is 

 to say, anterior and superior to the antennules. Another 

 interesting peculiarity about the facial region of the cara- 

 pace is that the basal joints of the antennas have coalesced 

 with the sternum of the antennary somite, and, consequent- 

 ly, that the bases of the antennae are immovable. There is 

 no vestige of a scaphocerite, and the aperture of the organ 

 which answers to the green gland of Astacus is provided 

 with a peculiar movable plate, provided with a projecting 

 internal stem, to which delicate muscles are attached in Car- 

 cinus. It is this structure which has been compared to an 



