DEVELOPMENT OF THE POLYPIDE. 41 



of Bugula. It should here be pointed out that the two different types of 

 development of the intestine are not fundamentally opposed to one another ; in 

 both cases a constriction pressing in from each side brings about the separation 

 of the enteric canal from the atrium. Whether the rudiment of the oesophagus 

 is completed at the same time is of comparatively little importance, and we 

 may well imagine, even in nearly related forms, that in some cases the first and, 

 in others, the second type of bud- development may be found. 



In Pectinella, on the contrary (Oka, No. 52a), in the statoblasts as well as in 

 the buds, the diverticulum that forms first is said to represent the common 

 rudiment of the oesophagus and the stomach, so that here the oral aperture 

 seems to form first. The intestine is said here to grow out as a caecum from the 

 stomach and to open towards the atrium (the upper cavity), the anal aperture 

 seeming to form in this way. 



The ganglion can be traced back to an invagination which forms at 

 the base of the upper cavity or atrium between the oral and the anal 

 apertures (Fig. 20 C, n). The lumen of this invagination gives rise 

 to the future brain-cavity. After the rudiment of the ganglion has 

 been completely cut off from the surface layer, an ear-shaped diverti- 

 culum forms on each side of it (Braem) ; these diverticula represent 

 the rudiments of the lophophoral nerves. Each grows out as two 

 nerves, one of which runs posteriorly into the corresponding arm of 

 the lophophore, while the other runs forward and spreads out upon 

 the oesophagus. 



The portion of the primary sac which remains after the intestine 

 has been separated from it is known as the atrium (Fig. 23 B, at) or 

 the cavity of the tentacular sheath. The greater part of its wall 

 becomes modified into the tentacular sheath. The rudiments of the 

 lophophore and of the tentacles (Fig. 23 B, at) develop early in 

 the base of this cavity. The first rudiment of the lophophore takes 

 the form of a swelling projecting into the atrium, and forming a 

 semicircular border to the oral aperture. In the Gymnolaemata, this 

 swelling closes in front of the anal aperture to form a ring, encircling 

 the mouth, which carries the tentacles. Corresponding to the 

 inwardly projecting swelling of the lophophore, there is, on the 

 outer side of the polypide-sac, a groove which soon changes into 

 a closed canal. This is the so-called circular canal, which must be 

 regarded as part of the body-cavity. The tentacles arise as out- 

 growths of the lophophore resembling the fingers of a glove. In 

 the Gymnolaemata they are situated in a row on either side of the 

 body (Davenport, Prouho). Only later do these two rows become 

 connected through the development of the tentacle-buds in front of 

 the mouth, while the lasfr tentacles to arise close the ring on the anal 

 side. 



