REPORT ON THE CRUSTACEA MACRTJRA. 



XVI! 



overlapped by a process from above and below, which covers it or encloses it within a 

 groove, as has been previously described. 



In Aristeus semidentatus the ophthalmus is hemispherical, and the peduncle 

 suddenly narrows and tapers to the base. In Penseus canaliculatus the ophthalmopod 

 is Particulate and laterally compressed, the joints articulating with each other 

 oblicpaely. It has rather a complicated appearance, having the ophthalmus situated 

 obliquely at its extremity, the inner surface of which is flat, with a concave margin 

 furnished with a small projecting pigmented process forming a connected imperfect 

 ocellus near the middle of the arch (Fig. III.). In Hepomadus glacialis the ophthal- 

 mopod is pear-shaped and flattened ; and at the angle formed between the cylindrical 

 and compressed portions there is a small papilla, A similar but more important papilla 



Fig. III. — Penceus canaliculatus. Ophthalmopod- 

 A inner, B outer surface. 



Flo. IV. — Gennadas intermedins. Ophthal- 

 mopod — a', ophthalmus ; a", ocellus ; gn, 

 optic ganglion. 



may be found in Benthesicymus and Gennadas, which culminates in some, if not in all 

 species into a prominently pointed tubercle, as shown in Fig. IV., having a small circular 

 lens at its extremity (PI. LVII. fig. 2>a ; PI. LVIII. fig. la), to which a distinct 

 branch of the optic nerve, originating in a ganglion at the base within the ophthalmopod, 

 may be distinctly traced. This secondary organ consists of a single lens, is very 

 translucent, and has no trace of pigment. It appears to be present only in deep-sea 

 specimens, but it is not confined to those of the Dendrobranchiate division, inasmuch 

 as a similar protuberance may be seen in Bentheocaris (PI. CXXIII. figs. 3a, 4a), 

 and in Hymenodora (PI. CXXXVII.) among the Phyllobranchiata. This circumstance 

 has induced me to believe that this pedicular ocellus may be an altered condition of the 

 sessile ocellus so common among the species of the latter division, and traces of which 



(ZOOL. CHALL. EXP. — PART LII. — 1888.) Fff C 



