22 Echinoderma. 



phoral or blood-lacuna r system consists partly of independent lacunae and 

 partly of spaces in the connective tissue of the middle zone. Chief among the 

 former are the internal and external lacunar systems of the gut. which are con- 

 nected with the interstitial lacunae in its wall and in the mesentery. The external 

 lacuna of Cucumaria Planci forms a large anastomotic trunk between the first and 

 second divisions of the gut, and also communicates with lacunae in the walls of 

 the genitalia by the problematic or genital canal. Both internal and external 

 lacunae are contained between the two layers of the mesentery and they communi- 

 cate posteriorly with the interstitial lacunae of the cloacal wall and gills , while 

 anteriorly they form a peripharyngeal system at the base of the water-vascular 

 ring. In Aspidochirotae the anastomotic trunk between the two parts of the gut 

 is connected with their internal lacunae ; but the two branches of the external la- 

 cuna also communicate by a network. The peripheral lacunar system in the middle 

 zone of the integument is connected with the external lacuna through the mesen- 

 tery, and also, through the pericloacal tracts, with the interstitial system of the 

 cloaca ; while the peripharyngeal lacunae place it in relation with the oral ring. 

 This gives rise to the radial lacunae, between nerve and water-vessel, with their 

 lateral branches to the tentacles and tube-feet. These radial lacunae, the mar- 

 ginal lacunae of the gut, and the genital canal are more definitely tubular than 

 those of the interstitial or schizocoel system, which never contain coagulum as the 

 former do. There is thus an imperfect differentiation into lymphatic and blood- 

 systems. But they are entirely distinct from the water- vascular system, though 

 the interstitial system communicates with the body cavity through stomata in the 

 peritoneal epithelium. The genital canal in Holothurians corresponds to the 

 lacunar system of the ovoid gland in Urchins, the gland itself is represented by 

 an areolar tissue full of amoebocytes, which is situated at the base of the water- 

 tube : while the mesoarium enclosing the latter is homologous with the oesophageal 

 mesentery in Urchins, and with the wall of the axial sinus in Asterids. The ar- 

 borescent organs or gills have no internal openings into the coelom. Their 

 chief function is a hydrostatic one, to fill up the coelom in the dilated animal. 

 But they are also respiratory and excretory, producing guanin, the caeca of their 

 left branch sometimes coming into relation with lacunar tufts on the intermediate 

 mesentery which represent glomeruli. They also seem to be lymphoid organs in 

 which amoebocytes are produced. The Cuvierian organs are not defensive 

 in any way, but simply modified gills adapted to glandular functions. - The 

 genital caeca of Colochirus Lacazii contain transverse but no longitudinal muscu- 

 lar fibres. The connective tissue beneath the internal epithelium contains 

 numerous amoebocytes from the genital canal. The caeca are developed laterally 

 from a mass of spherical cells situated in the connective tissue of the dorsal 

 mesentery along the line of the genital canal. The mid- ventral radius of a 

 Holothurian represents the left ventral radius of an Urchin, so that the madre- 

 poric interradius of the Urchin is in the mid-dorsal line of the Holothurian. 



Ludwig (*) points out that the viviparous Holothurians with marsupial 

 pouches which Lamport referred to Cucumaria crocea really belong to C. laeci- 

 gata, so that his observations do not in any way clash with those of Wyville 

 Thomson [see Bericht for 1886 Ech. p 12]. The dorsal ambulacra of C. crocea 

 become modified and their tissues swollen before the eggs are laid. The tube- 

 feet are not free, but remain included in the skin and fuse together, so as to form 

 longitudinal folds, on the surface of which are rows of little pits, marking the 

 sucking disks of the feet. Lampert confirms these observations, and redescribes 

 the marsupial pouches in the two ventral interambulacra of C. laevigata, but has 

 still failed to discover any connection between these and the ovaries. 



