ANNULATA. 225 



Order Holothurioidea (Sea-cucumbers). 



Bodi/ vermiform ; integument flexible, with scattered reticulate 



calcareous corpuscles, and a rudimental " lantern." Intestinal canal 



convoluted, vent terminal Respiratory organ, an anal ramified tube. 



Tstmilj IIoLOTHUEiiD^. Genera Holothuria, Pentactes, Bohad- 



schia, Cladolabes. 

 Synaptid^. Genera S7/?iapta, Chirodota. 



Order Sipunculoipea. 



Body vermiform; integument without calcareous corpuscles, and not 

 perforated by tubular tentacles. Intestinal canal convoluted, vent at 

 the fore part of the body. No respiratory organ. 



Family Sipunculibm. Genera Sipunculus, Phascolosoma. 

 EcmuRiBM, Genera Echiurus. Thalassema. 



LECTURE XL 



ANNULATA. 



In both the infusorial and entozoic classes the body assumes a more 

 perfect linear and bilateral form as the species advance in the scale 

 of organisation ; and we hare seen in the subjects of the preceding 

 discourse, that even the typical radiated class of the zoophytic 

 province conducts by the holothurian and sipuncular families to the 

 vermiform class of the articulated province, in which the vegetative 

 principle of development, by the frequent repetition of similar parts, 

 is still conspicuously manifested, but exercises its energies in a linear 

 direction, and forms successive segments from before backwards. 

 We find, in fact, at the lowest step of the great homogangliate series 

 of the Animal Kingdom an extensive group of vermiform animals, 

 some of which very closely resemble the trematode, and others the 

 nematoid, Entozoa, and all are devoid of jointed limbs : but they 

 possess a distinct circulating system of arteries and veins, and in 

 almost all the species the blood is red.* They have therefore been 



* The first notice of this colour of the blood in an anellid I believe to be due 

 to Pallas : — 



" Ibi trans cutem distincte apparet stria albicans — quam vena tequalis, rubra, 

 maximam partem sinisterior comitatur." 



" Intestiuo prune longitudinahter exterius adnascitur ductus crassus, subflexuosus, 



