574 COMPARATIVE ANATOMY CHAP. 



these folds lie close to one another, being only separated by a limiting 

 membrane containing vessels. According to the courses of these 

 vascular folds which project into the collar ccelom, the Enteropneusta 

 can be divided into two groups. 



G roU p i Balanoglossus, Glandiceps, Schizocardium. A fold commences at the 

 posterior end of the ccelom on each side, near the ventral median line, and ascends 

 diagonally in a curve anteriorly to the neck of the proboscis. 



Group 2 Ptychodera. A medio-ventral vascular fold runs anteriorly from the 

 posterior end of the collar region, dividing at a short distance from the anterior 

 end of that region into two folds, which ascend perpendicularly and encircle the 

 buccal cavity. 



The walls of the collar coelom are for the most part developed as 

 muscles. 



1. The parietal wall consists first of an outer layer of longitudinal muscle 

 fibres. These commence, it is true, posteriorly in the visceral wall, then run slant- 

 ingly forwards and outwards towards the integument, traversing the coelom. Only 

 in the anterior collar region do they run close under the integument to the anterior 

 end of the collar. Only in this anterior region of the collar also is a circular 

 muscle layer developed on the inner side of the longitudinal musculature. 



2. The visceral wall contains first an inner longitudinal musculature, according 

 to the arrangement of which the Enteropneusta may be divided into two groups. 



Group 1 Schizocardium, Glandiceps, Balanoglossus. A bundle of longitudinal 

 fibres rises on each side anteriorly, from the proboscidal skeleton, which will be 

 described later. These muscles spread out fan-like towards the septum between the 

 collar and the trunk. The fibres composing this fan slope more and more the 

 nearer the ventral middle line they become attached to that septum. Anteriorly, the 

 two bundles of fibres surround the efferent vessel of the collar. 



Group 2 Ptychodera. The numerous bundles of longitudinal fibres run 

 parallel to one another. Only a few of them, viz. those lying nearest the median 

 line, run as far as the neck, enclosing the efferent collar vessel and becoming 

 attached to the skeleton of the proboscis. None of the rest reach the anterior 

 end of the collar region, but become attached to the posterior wall of the vascular 

 fold mentioned above as encircling the buccal cavity. 



The visceral wall of the collar coelom further consists of a transverse musculature. 

 This also is differently arranged in the groups mentioned above, its distribution 

 being determined by the courses of the vascular folds. 



In group 1 (Schizocardium, Gflandiccps, and Balanoglossus) the transverse fibres 

 run out laterally from the dorsal point of attachment of the ventral mesentery on 

 each side and then upwards, to become attached to the vascular fold above described 

 which runs from behind and below, anteriorly and upward. 



Where the ventral mesentery is wanting, the transverse fibres run without 

 interruption from the right to the left vascular fold, surrounding the buccal cavity 

 ventrally. The transverse fibres are quite short posteriorly and limited to the 

 ventral side, for here the vascular folds are but commencing to diverge. Anteriorly, 

 as the folds gradually meet over the buccal cavity, the fibres become longer and 

 longer ; they finally form, near the insertion of the neck, circular loops almost 

 completely surrounding the buccal cavity, these loops being interrupted for only a 

 short distance dorsally. 



In group 2 (Ptychodera} this transverse musculature is altogether wanting 



