THE EVIDENCE OF THE RESPIRATORY APPARATUS l6l 



The Eespiratory Chamber of Ammoccetes. 



In order to make clear the nature of the branchial segments in 

 Ammoco?tes, I have divided the head-part of the animal by means of 

 a longitudinal horizontal section into halves — ventral and dorsal — 

 as shown in Figs. 63 and 64. These figures are each a combination 

 of a section and a solid drawing. The animal was slit open by a 

 longitudinal section in the neighbourhood of the gill-slits, and each 

 half was slightly flattened out, so as to expose the ventral and dorsal 

 internal surfaces respectively. The structures in the cut surface were 

 drawn from one of a series of horizontal longitudinal sections taken 

 through the head of the animal. These figures show that the head-region 

 of Ammoccetes consists of two chambers, the contents of which are 

 different. In front, an oral or stomodseal chamber, which contains the 

 velum and tentacles, is enclosed by the upper and lower lips, and was 

 originally separated by a septum from the larger respiratory chamber, 

 which contains the separate pairs of branchiae. A glance at the two 

 drawings shows clearly that Eathke's original description of this 

 chamber is the natural one, for he at that time, looking upon Ammo- 

 ccetes branchialis as a separate species, described the branchial chamber 

 as containing a series of paired gills, with the gill-openings between 

 consecutive gills. His branchial unit or gill, therefore, was repre- 

 sented by each of the so-called diaphragms, which, as seen in Figs. 63, 

 64, are all exactly alike, except the first and the last. Any one of 

 these is represented in section in Fig. 65, and represents a branchial 

 unit in Eathke's view and in mine. Clearly, it may be described as a 

 branchial appendage which projects into an open pharyngeal chamber, 

 so that the series of such appendages divides the chamber into a 

 series of compartments, each of which communicates with the exterior 

 by means of a gill-slit, and with each other by means of the open 

 space between opposing appendages. 



Each of these appendages possesses its own cartilaginous bar 

 (Br. cart.), as explained in Chapter III. ; each possesses its own bran- 

 chial or visceral muscles (coloured blue in Figs. 63 and 64), separated 

 absolutely from the longitudinal somatic muscles (coloured dark 

 red in Figs. 63 and 64) by a space (*S^>.) containing blood and 

 peculiar fat-cells, etc, Each possesses its own afferent branchial 

 blood-vessel from the ventral aorta, and its own efferent vessel to 

 the dorsal aorta (Fig. 65, a. br. and v. br.). Each possesses its own 



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