60 THE TREMA TO DA 



characters of the order. Aspidogaster, v. Baer, in Mollusca. Platyaspis, 

 Montic., in Chelonians. Cotylogaster, Montic., in Cantharus vulgaris. 

 Macraspis, Ols., in gall bladder of CJtimaera. 1 



Further Remarks on the Order Aspidocotylea. The members of 

 this group agree in their life -history with the monogenetic 

 Heterocotylea, but structurally they are more closely related 

 to the Malacocotylea, especially in the arrangement of the ex- 

 cretory and the reproductive organs, as well as in their endo- 

 parasitic habit. In their young condition, with a small posterior, 

 ventral sucker and an anterior oral sucker, Aspidogaster very 

 closely resembles Distomum. The form of the alimentary canal 

 and details in the generative organs are, however, peculiar to the 

 order. It has been by some authors grouped with the Hetero- 

 cotylea (Monogenea), by others with the Malacocotylea (Digenea). 



FIG. VI I. Aspidogaster. 



The upper figure represents a side view of 

 the animal, a, mouth ; b, pharynx; c, intes- 

 tine ; d, genital pore in the cervico-pedal pit 

 (of Stafford) ; /, foot carrying the sucker ; q, 

 ventro-lateral nerve ; r, cerebral ganglion ; t, 

 body ; x, excretory pore. 



The :lower figure is a diagrammatic trans- 

 verse section, at about the level of the ger- 

 niarium. a, the horizontal muscular septum 

 dividing the body cavity filled of course with 

 parenchyma into an upper chamber, and a 

 lower, and extending from the genital pore to 

 the hinder end of the intestine ; b, the collect- 

 ing vessels of the excretory system ; c, intes- 

 tine ; d, excretory tubule ; /, uterus ; g, vitel- 

 lariuin ; fc, germarium ; p, penis ; q, longitudinal 

 nerve in the foot ; s, sucker showing the three 

 ridges and the margin. 



But Burmeister first, Monticelli more recently, have erected a special 

 order for it and its allies. 



Aspidogaster conchicola, v. Baer, is a small organism occurring 

 in various species of Unio and Anodon of Europe and North 

 America. According to Yoeltzkow it infests several organs, but 

 most frequently (66^/) Keber's red-brown organ; less frequently 

 (3 3^) the pericardium, and very rarely the organ of Bojanus ; 

 whilst in the young stages it is parasitic in the intestine. It also 

 occurs in some fresh-water Gastropods. 



The general shape of the animal, with its neck, body, and 

 central sucking disc, recalls strongly that of some Gastropod 

 mollusc (Fig. VII.). The general anatomy will be gathered 

 from the diagrams (Fig. VIII.) ; it is a form comparatively easy 

 to obtain and to study. 



1 It is probable that Stichocotyle, Cunningham, belongs to this family, rather 

 than to the Holostomidae (see 36). It is only known in an immature condition encysted 

 in the wall of the intestine of Homarus americanus and Nephrops norvegicus. Monti- 

 celli also includes Aspidocotyle in this order. 



