

8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



parisons with other species where the male apparatus in known only 

 in the resting condition, have increased the difficulties of deciding 

 whether one is here dealing with a new species or not. The great 

 resemblance, even almost the identity, of the eye arrangement of the 

 present species with that of the common Mediterranean Prosthiosto- 

 mum, P. siphunculus (Delle Chiaje) 1828, fully described by Lang, 

 1884, will not have escaped the notice of students of polyclads. On 

 the other hand, it seems highly improbable that the same species would 

 be found in two such widely separated localities as the Mediterranean 

 and the Galapagos Islands. It further appears that P. siphunculus is 

 very much larger than P. parvicelis. According to Lang, 1884, the 

 former may reach a length of 30 mm. and the majority of the sexual 

 individuals range from 12-18 mm. The specimen of P. parvicelis is 

 already fully mature at a length which could not have been more than 

 8 mm. in the extended condition. The principal difference, however, 

 between the two species, concerns the shape of the seminal vesicle and 

 the points of entrance of the vasa deferentia. Lang, 1884, plate 30, 

 figure 20, shows the seminal vesicle of P. siphunculus as curved, with 

 a simple fusiform lumen and muscular wall of uniform width ; the 

 vasa enter at its proximal end. In P. parvicelis the wall of the seminal 

 vesicle is very thick proximally and thins out distally, the lumen is 

 of peculiar shape (fig. 15), and the vasa deferentia enter the sides 

 and then turn proximally to join the beginning of the lumen. The 

 shape of the cement pouch appears also to differ in the two forms. On 

 these grounds I with some hesitation consider the Galapagos species 

 distinct from the Mediterranean one. 



Differential diagnosis. — Eye arrangement as in P. siphunculus; 

 smaller than this species ; seminal vesicle with thicker wall and small 

 lumen proximally, thin wall and larger lumen distally, receiving the 

 vasa deferentia into its sides. 



Locality. — Sulivan Bay, James Island, Galapagos Islands, shore 

 collecting. Collected by Dr. Waldo L. Schmitt, July 24, 1938. 



Holotype. — One whole mount, U.S.N.M. no. 20426. 



REFERENCES 



Bock, Sixten 



1913. Studien iiber Polycladen. Zool. Bidrag, vol. 2, pp. 1-343. 

 1923. Polycladen aus Juan Fernandez. The Natural History of Juan 

 Fernandez and Easter Island, vol. 3, pp. 341-372. 

 Girard, Ch. 



1893. Recherches sur les Planaires et les Nemertiens de l'Amerique du 

 Nord. Ann. Sci. Nat., vol. 15, No. 7, pp. 145-310. 



