398 INVERTEBBATA CHAP. 



cells eventually form a coherent layer, but appear to arise as 

 wandering cells which adhere individually to the external surface 

 of the polypide. From this mesodermic layer the coelomic canals 

 of the tentacles and the ring canal which unites them, are derived. 

 The constriction between gut rudiment and atrial rudiment becomes 

 so deep at one place as to completely sever the two from each other, 

 but before and behind this place two openings are left by which 

 the two rudiments still communicate, and these openings form the 

 mouth and anus of the new person. The gut rudiment becomes 

 divided by constrictions into oesophagus, stomach, and intestine. 

 The atrial rudiment develops the lophophoral tentacles as ridges 

 projecting into its cavity, and at the completion of development it 

 reacquires an opening to the exterior. The retractor muscles, 

 fuuiculus, etc., are derived from scattered mesoderm cells, which 

 originate from the mesoderm cells of the mother. 



Bonier, who investigated the buds of Alcyonidium, differs only 

 from Seeliger in finding that in Alcyonidium the polypide passes 

 from the condition of a solid thickening to a closed sac at one step, 

 and in finding mesoderm cells produced by budding from the 

 ectoderm as well as from the maternal mesoderm cells. While this 

 is possible we may conclude that it is unlikely. 



POLYZOA ENTOPKOCTA 



The group of Polyzoa Eutoprocta comprises only three genera, 

 viz. Loxosoma, Pedicellina, and Urnatella, and the complete life-history 

 has only been worked out in the case of Pedicellina. We shall 

 select as type for special study Pedicellina echinata, the early develop- 

 ment of which was worked out by Hatschek (1877) whose results 

 were confirmed and extended by Lebedinsky (1905). The remark- 

 able metamorphosis undergone by the larva was elucidated by 

 Harrner (1887). 



PEDICELLINA ECHINATA 



The egg is fertilized whilst it is still in the ovary, and is pene- 

 trated by several spermatozoa, but only one unites with the nucleus 

 while the rest are absorbed. The egg is then dehisced into the 

 atrium of the parent, where it is retained until it has developed into 

 a full-grown larva. 



The egg segments somewhat unequally, the blastomeres at the 

 animal pole being much smaller than those at the vegetable pole. 

 Already when seven blastomeres have been formed a blastula stage 

 has been reached, the upper half being formed of three small 

 blastomeres, the lower of four larger ones ; the blastocoele is slit-like. 

 When, however, segmentation has been completed, there results a 

 spherical blastula whose lower cells are larger and have larger yolk- 

 granules than the upper cells. Where the border between these 

 two kinds of cell is situated, there are to be found two cells distinctly 



