49] LIFE HIS TOR Y OF TREMA TODES—FA UST 49 



Braun (1893:683) considers the ventralis, dorsalis, and pharyngealis to be 

 the three pairs of posterior nerves. However, the consistent course of the 

 lateralis to the posterior extremity of the body, its early appearance 

 in the embryology of the worm, and its commissural connections with the 

 anterior lateralis and posterior dorsalis surely prove its right to a place in the 

 rank of the primary posterior nerve trunks. 



The nervous system of the monostomes has been worked out by Jager- 

 skiold (1891) for Ogmogaster pUcatus (Crepl.), and by Monticelli and Looss 

 for Catatropis verrucosa (Frol.). Looss (1896:149) considers the system similar 

 to the distome t>'pe. "Je n'hesite pas a attribuer au systeme nerveux de 

 notre ver une construction analogue a celle que nous avons deja signalee chez 

 un bon nombre de Trematodes digeneses. " However, Looss (1896:11-16) 

 was not able to make out clearly the anterior trunks. Jagerskiold (1891 :14-16) 

 describes the cerebral ganglion masses with the transverse commissure, the 

 posterior ventralis, dorsahs, and lateralis, and the three anterior trunks, short 

 and thorn-like, the homologs of the dorsalis, lateralis, and ventralis. A stem, 

 designated as the "fourth," arising from the anterior reaches of the cerebral 

 masses, passes ventrad to the region below the oral sucker; it seems probable 

 that it is the palatinus. The only real modification from the distome type 

 is the absence of the acetabular innervation, due to the loss of this organ. 



In the Amphistomata adults among the earlier writers Lejtenyi (1881:142- 

 144), working on Gastrodiscus polymastos Leuck., described two ganglion centra 

 with the dorsal commissural bridge, but witli only one anterior and one poster- 

 ior pair of trunks. In contrast to this incomplete description is that given 

 by Looss (1896:21, 22) for Gastrodiscus aegyptiaciis (Cobb), where the usual 

 distome nerve trunks were found, and, in addition, a median anterior and a 

 median posterior nerve. Looss has worked out the nerve anatomy for Am- 

 phistomum suhclavatum Rud. in even greater detail (1892:151; Taf. 19, 

 Figs. 1, 2, ), and finds that they correspond to the distome type, except for 

 the innervation of the posterior sucker. 



The one group of the Digenea where the nervous system has been almost 

 entirely neglected is the Holostomata. Brandes (1891) states that none of 

 the workers of the nervous system up to his day have worked on the holosto- 

 mids. He has observed only the central nerve center lying above the posterior 

 portion of the pharynx, an anterior and a posterior pair of nerve trunks, the 

 tracings of which he has found in sections. Thoss (1897), working on Holo- 

 stomum cucullus, finds the main nerve center lying dorsal to the origin of the 

 esophagus, with two pairs of anterior and one pair of posterior nerve trunks. 



The central nervous system of Cercaria trisolenata, the echinostome, 

 consists of two masses of ganglion cells and the dorsal commissure lying con- 

 cavely on the dorsal side of the large muscular pharynx. The commissure is 

 broad and flat. The dorsal surface of the ganglion masses and the commissure 

 present a smoothly curved surface, but on the ventral side the ganglia bulge 

 out against the pharynx. The anterior trunks consist of the dorsalis, lateraUs, 

 ventralis, and the palatinus. The paired dorsales arise together with the 



