438 Prof. M' 111 tosh on the 



ward and inward. The foregoing arrangement thus holds 

 the whole body-cavity in control, and, like the complex 

 muscles of the vertebrate heart, must materially aid the 

 blood-vessels in propelling the blood forward into the 

 anterior region and in keeping it there — during the con- 

 traction of the other muscles of the part — by firm closure of 

 the channel of communication. Its importance, therefore, 

 in regard to the functions of the proboscis is apparent. 



With the diminution and atrophy of the transverse 

 ventral muscles over the nerve-cords in the posterior region, 

 the vertical and oblique also coincide, so that by-and-by 

 only a few slender fasciculi of each remain — stretched from 

 the dorsal raphe in the case of the vertical and the lateral 

 wall in the oblique, to the atrophied transverse on each side 

 of the ventral blood-vessel. Toward the extremit}^, again, 

 both become well developed, the oblique spreading from 

 the outer part of the arch over the nerve-cord upward and 

 outward in a fan-like manner to the body-wall ; while the 

 vertical extend from the median part of the arch outward 

 and then upward to the exterior of the dorsal longitudinal 

 muscles. 



As soon as the under lip of the animal becomes continuous 

 beneath the extruded proboscis, a series of transverse fibres 

 pass — from the raphe at the inner side of each longitudinal 

 ventral muscle — quite across the body, and thus form a very 

 efficient expulsive system, as well as a harrier to return after 

 extrusion. It is probably this muscle which is observed in 

 the living animal, after expulsion of the proboscis, stretched 

 as a very strong transverse band or arch between the bases 

 of the tentacles, apparently blocking (with the pre-oral, 

 transverse and vertical) the passage of blood forward into 

 the cephalic contractile space, and forming an abutment 

 against the posterior part of the vascular sinus behind the 

 triangular region. When the animal is viewed from the 

 ventral surface (the proboscis being withdrawn) the fibres 

 pass right across the body- wall in front of the first bristle- 

 bundles. Behind the latter they become soniev^hat indistinct, 

 but at the third pair they are very evident — extending in- 

 ward and slightly backward to the central raphe. They are 

 broad in front and gradually diminish toward the posterior 

 end of the region. This ventral transverse muscle continues 

 for some distance backward as a powerful layer, and then 

 presents a chitinous raphe in the median line, so as to form 

 two muscles. Throughout the anterior region the latter 

 stretch from the common raphe externally to the ventral 

 median line, under the hypodcrm, and they are sometimes 



