II. A FAUNA WITHIN ANIMALS. 49 



§ 6. History, Structure, etc., of Streptostomum, and Thelastomum. 



In Thelastomum, the body is longer, has shorter and more numerous annulations, 

 and a shorter tail than in Streptostomum. In the former, also, the buccal organ is 

 relatively very long, and cylindrical, in the latter short and pyriform. In Oxyuris 

 the body is relatively longer than in either the above, is more shortly and nume- 

 rously annulated, and has the tail constructed like that of Ascaris, instead of being 

 straight and spiculate, as in the former genera. 



In Ascaris and Oxyuris the tail appears to be formed by the gradual attenuation 

 of the body posteriorly to a point; in Streptostomum and Thelastomum it has more 

 the appearance of being a superadded appendage, even in the species Thelastomum 

 irevicaudatum, which has comparatively a very short tail. 



In the two genera last mentioned, the tegumentary envelop of the body is so 

 transparent that all the interior structures are distinctly visible. The integument 

 is lined by two layers of muscular fibres, the first transverse, the second and stronger, 

 longitudinal. 



"Within the muscular investment, apparently upon one side only, in Strepto- 

 stomum just posterior to the commencement of the intestine (PI. VII. 2, e), and in 

 Thelastomum at the termination of the buccal organ (1, e), is an apparent follicle, 

 communicating with the exterior, and having its bottom connected by means of 

 radiating bands of fibres to the external surface of the alimentary canal in its 

 vicinity. 



Along the course of the interior ventral surface are situated several other appa- 

 rently glandular organs (PL VI. 4, e). 



The intimate structure of the alimentary canal presents nothing different from 

 that given in the account of Ascaris infecta. 



A remarkable special peculiarity in the construction of the intestine is observable 

 in Thelastomum appendiculatum. In all other species of the genus, and in Strepto- 

 stomum, the intestine is simply cylindroid, straight throughout, and more or less 

 dilated into different forms at 'its commencement. In Thelastomum appendiculatum 

 (PL VII. 3), the intestine commences by a broad, deeply sinuate cordiform dilata- 

 tion, which rapidly narrows to a short cylindroid portion and then sends off a 

 long, capacious, gourd-form receptacle, or diverticulum {d), and afterwards pro- 

 ceeds backwards to the rectum, and in its course, in the vicinity of the generative 

 aperture, performs a single short convolution. 



The generative apparatus (PL VI. 4) of Streptostomum, and Thelastomum is con- 

 structed upon the same plan as that of Ascaris infecta, except that there is no sper- 

 matheca, and the dilatable portions of the oviducts terminate at once in the vagina, 

 which passes obliquely backward, and ventrally to the generative aperture, (c?) 



