BRIAKOSACCUS CALLOSUS, A NEW GENUS AND NEW 

 SPECIES OF A RHIZOCEPHALAN PARASITE OF LITH- 

 ODES AGASSIZII SMITH 



By H. BoscHMA 



Of the University in Leiden, Holland 



The collection of the United States National Museum contains 

 among a large number of other specimens, which for the greater part 

 are representatives of previously described genera, one that is remark- 

 able for its enormous size. It is a parasite of Lithodes agassizii Smith, 

 which has been taken from its host. Consequently its position on 

 the host is unknown, but probably the long axis of the parasite was 

 lying in the transverse plane of the host, as in this manner more space 

 is available between the thorax and the abdomen of Lithodes. 



The internal anatomy of the parasite is very similar to that of 

 Peltogaster, and the parasite certainly is a representative of the 

 Peltogastridae. In some respects, however, the specimen differs from 

 Peltogaster and the other known genera of the family, and these dif- 

 ferences are striking enough to establish for this specimen a new 

 genus, which I propose to call Briarosaccus on account of the gigantic 

 size of the type specimen. This genus may be defined as follows: 



BRIAROSACCUS, new genus 



Body slightly elongate, curved. Stalk about in the central part of 

 the dorsal surface. Mantle opening at one extremity. Mesentery 

 from the mantle opening to the posterior part of the dorsal surface, 

 thin, except in the region of the stalk. Colleteric glands at the left 

 and right side of the mesentery, highly lobular. Testes paired, situ- 

 ated in the dorsal part of the visceral mass, parallel to long axis. 



In general structure Briarosaccus reminds one strongly of that of 

 Peltogaster, from which it differs chiefly in the relative narrowness of 

 the mesentery, the more complicated structure of the colleteric glands, 

 and in its peculiar retinacula. The mesentery which joins the 

 mantle to the visceral mass in Peltogaster is very broad (Boschma, 

 1928, fig. 4), having the breadth of one-fourth or more of the surface 

 of the latter. Consequently it can not be compared directly with 

 that in the Sacculinidae, in which there is a narrow ligament connect- 

 No. 2804.— Proceedings U. S. National Museum. Vol. 76. Art. 7 



93740—30 1 



