POLYPIFERA. 



future alimentary canal, the interspace being 

 in communication with the stem of the parent 

 polype, and filled with a fluid that is analogous 

 to the blood of the higher animals ; superiorly 

 this cavity likewise penetrates into the ten- 

 tacles, and the fluid which bathes the exterior 

 of the alimentary canal thus finds admission 

 even to the extremities of those organs 

 (/g. 62. 6, m). 



Fig. 62. 



Development by buds of Laguncula repens. 



1. A young bud, with the cavity of the stalk ex- 

 tending into it, the parietes thickened at one part ; 

 it is from this part that the intestinal canal and 

 tentacles are formed. 



2. A sac is formed in the interior, having a longi- 

 tudinal fold, which afterwards becomes the tentacles. 



3. The rudiments of the tentacles are here appa- 

 rent : the view is taken so that the space which they 

 surround is visible. 



4. In this figure all the organs of the animal can 

 be distinguished, but the bud is not yet opened. 



5. Here the cell is opened, and the animal is ready 

 to expand itself. 



6. Section of the adult animal ; a, tentacles ; 6, 

 mouth ; c, buccal cavity ; rf, valve separating this 

 cavity from the oesophagus ; e, oesophagus ; f, sto- 

 mach; g, pyloric cilia; A, pyloric valve; i, intes- 

 tine ; k, anus ; I, peri-intestinal cavity ; m, commu- 

 nication of this cavity with the interior of the ten- 

 tacles; n, nervous ganglion; o, long retractor 

 muscle ; p, retractor of the stomach ; q, walls of the 

 cell. ( After Van Beneden.) 



The second cavity, which is the intestinal, 

 has as yet no communication with the ex- 

 ternal world. As the formation of the ten- 

 tacula proceeds, the portion which is situated 

 in front of them will become the sheath, and 

 the other part the proper intestinal canal; 

 the former cavity is, therefore, in all respects 

 comparable to that which exists in the Tuni- 

 cata situated in front of the proper oral orifice 

 and lined with the branchial vessels. The 

 tentacles of these polypes, in fact, if connected 

 by transverse canals and attached to the 

 sheath, would transform the animals in this 

 phasis of their growth into Ascidians. 



As the tentacula are formed by the pro- 

 longation of the tubercles which were their 



first rudiments, the cavity of the stomach and 

 the rest of the intestinal tube gradually be- 

 come apparent, and at the same time some 

 globules are visible disposed around the cul- 

 de-sac of the former viscus, which gradually 

 become arranged into fibrillae, and constitute 

 the retractor muscles. 



At what time the nervous system is formed 

 could not be detected. 



When the cell has nearly reached its full 

 development, the tentacular sheath is com- 

 pleted in the same proportion, the parietes of 

 the cell become softened, and an opening is 

 formed which brings the young polype into 

 communication with the surrounding element. 

 The Bryozoon has now attained its full deve- 

 lopment, and can expand its tentacula, but as 

 yet there are no traces of the reproductive 

 organs, which seem to be formed after all the 

 others. 



In Halodaclylus reproduction by gemma- 

 tion is effected by the development of young 

 animals and cells amongst the mature ones. 

 The newly formed cells are triangular, and 

 the animal looks like a mere spot in their 

 centre. As they grow they thrust aside the 

 surrounding cells, and the number of their 

 sides increases until they acquire the regular 

 hexagonal form of the adult. 



The Halodactylus likewise afforded Dr. 

 Farre an opportunity of witnessing the second 

 mode of reproduction common to the Bryozoa, 

 namely, by the development of ciliated gem- 

 mules. These are readily seen in Spring as 

 minute whitish points situated just below the 

 surface of the mass (./%. 64, a). Sometimes 



Fig. 63. 



Thin transverse section of Halodactylus diaphonus. 

 The centre occupied by cellular tissue and water. 

 The circumference formed by cells in close apposi- 

 tion. The brown bodies scattered through the sub- 

 stance. 



a, a, position of the gemmules, enclosed in the 

 sac ; b, one of the gemmules escaped during the sec- 

 tion into the central tissue. (After Farre.} 



they are of a darker colour, and exceedingly 

 numerous, appearing to .occupy almost its 

 whole substance. If one of these points be 

 carefully turned out with a needle, it is found 



