COE: NEMERTEANS OF WEST AND NORTHWEST COASTS. 55 



most species of hoplonemerteans the ocelli are very tiglily special- 

 ized, and are often sufficiently large to be seen with the unaided 

 eye. They are often numerous, and are of fairly constant number 

 in different specimens of equal size of the same species, but differ- 

 ent members of the genus have a widely different number of ocelli. 

 Amphiporus hiocidatus^ for example, has but a single pair, while 

 some individuals of A. forinidabUis (Text-fig. 23 ; PI. 17, fig. 101), 

 have upward of 250, and in several species of the same genus ocelli 

 are entirely wanting. When ocelli are numerous they increase 

 rapidly in ni;mber during the growth of the individual (as in A. 

 impai-ispinosus). In Drepajs^ophorus (Text-fig. 22 ; PL 24, fig. 

 181) the ocelli reach their highest development, each ocellus con- 

 sisting of a complete visual apparatus including lens, pigment cells, 

 retina, and optic nerve. 



In Tetrastemma 4 ocelli are usually present, and these are 

 arranged on the head at the corners of a square or rectangle (Text- 

 fig. 21 ; PI. 2, figs. 17, 18, 25 ; PL 17, fig. 104 ; PL 18, fig. 113 ; PL 

 22, figs. 145, 146). In T. aherrans, how^ever, the ocelh are much 

 fragmented (PL 24, fig. 150), while in T. caecum they are entirely 

 wanting. 



In the Pacific coast species of Amphiporus the smallest number 

 of ocelli in mature individuals of any species is 8, while in another 

 form {A. formidabilis) as many as 250 may be present. The aver- 

 age number in the 17 species from this region in which the ocelh 

 have been accurately observed is approximately 40, and this is 

 about the average numlier in species from other parts of the w^orld. 

 When few" in number they may be arranged in a single row on each 

 margin of the head, as in A. cruentatus (Text-figs. 20, 51 ; PL 1, fig. 

 13), but when very numerous are usually distributed in two or more 

 elongated clusters on each side (Text-figs. 23, 46, 53 ; PL 16, figs. 

 93, 95, 97, 99; PL 17, fig. 101 ; PL 18, fig. 117 ; PL 22, figs. 154, 

 157 ; PL 23, figs. 162, 163; P1..25, figs. 195-199). 



Emplectoxema has very numerous, but minute, ocelli; Zygoxe- 

 MERTES has upward of 50, several of which are scattered in a single 

 row along the lateral nerv'es for some distance beliind the brain (PL 

 22, fig. 141 ; PL 24, fig. 182). In Paranemertes californica there 

 is but a single pair of minute ocelli, while the other three species of 

 the genus have a considerable number (PL 23, fig. 177 ; PL 24, fig. 

 190; PL 25, figs. 198, 199). In Nemertopsis there are 4 eyes of 



