II. Pelinatozoa. 



II. Pelmatozoa 



(including Crinoidea, Cystidea, and Blastoidea). 



See also Perrier (*), supra, p 3, Fraas, supra, p 4, Sturtz f 1 , 2 ), infra, p 9. 



According to Barrois (S 2 ) the real blastopore ot Comatula does not per- 

 sist, but its closure accompanies the formation of mesenchym-cellules at the ex- 

 pense of the endoderm. The archeuteron divides into two parts, an anterior 

 which gives rise to the water- vesicle and intestine , and a posterior which en- 

 larges to form the two peritoneal sacs. These grow round the intestine, but give 

 off no processes either backwards or forwards. The central cord of the stem is 

 exclusively mesenchymatous in origin. The tentacular vesicle forms at the ex- 

 pense of the supposed blastopore. This is a pit which appears late and is invagi- 

 uated after the larva is fixed to form a closed sac which passes between the am- 

 bulacra! ring and the ectoderm of the oral dome, and receives the tentacular 

 extensions of the former. It is eventually opened to the exterior by the rupture 

 of the oral dome. The calycular portion of the larva is really posterior, and the 

 peduncular portion anterior, having only a temporary existence and representing 

 a p re-oral lobe. The Pluteus of Echinids has a similar structure which degene- 

 rates : while the body of the Pluteus, representing the posterior part of the larva, 

 becomes the young Urchin. The amuios of MetschuikofF is homologous with 

 the tentacular vestibule in the Crinoid. 



The elaborate memoir of PerriGP ( 2 ) on the development of Antedon which is in 

 course of publication in the Archives of the Paris Museum has also been issued 

 separately in a complete form. Rather more than Y 3 of the volume is occupied 

 by a critical and historical study of the literature of the subject. The earliest 

 larva observed was at the period of fixation , with about 9 stem joints. Both 

 visceral and parietal layers of the peritoneal sac share in the formation of the 

 axis of the stem, though Barrois ( J j denies this. The primary water-tube, de- 

 pending from the water-vascular ring , communicates directly with the exterior, 

 and not indirectly through the body-cavity as stated by Ludwig [see Bericht for 

 1880 I p 254]. The parietal layer of the peritoneal axis of the stern gives rise 

 at an early period to 5 cellular cords which become hollow and enlarge into the 

 peripheral cavities of the chambered organ. Its nervous envelope is formed from 

 the surrounding mesoderm. At the moment of liberation from the stem there are 

 5 water-tubes, each communicating directly with the exterior by a ciliated funnel, 

 the water-pore. Later on, however, the pores and tubes do not correspond in 

 number. The canals described as vessels by Ludwig and Carpenter do not con- 

 stitute a true circulatory apparatus [see Bericht for 1885 I p 1861. Many of 

 them are continuous with the water- tubes ; while others open externally by water- 

 pores. Together with the water- vessels . they constitute a vast irrigation appa- 

 ratus, which contains water, not blood. It consists of 5 important parts. 1) In- 

 testinal network, 2) genital plexus, 3) labial plexus, and 4) the basilar circle 

 which joins the central ends of 5) the coeliac canals. Of these 3 is in connection 

 with the water-vessels and with the exterior. It developes as an extension of the 

 parietal layer of the peritoneum beneath the primary water-tube, which gradually 

 extends round the gullet and forms a perioesophageal canal. This opens into a 

 sac formed from the intraparietal portion of the water-tube , and thus communi- 

 cates both with the exterior and with the water-vascular ring ; and it is even- 

 tually developed into the spongy organ of P. H. Carpenter [see Bericht for 

 1885 I p 1861- The dorsal organ is not a simple structure but consists of two 

 parts. 1) The genital stolon in which the genital cords of the arms originate, 



