cxxxii Introduction. 



into which a circular vessel surrounding- the oesophagus, and fur- 

 nished with contractile ampullae^ sends prolongations. Though 

 this vascular system has no locomotor feet, like those of the normal 

 Echinodermata, in connection with it, yet_, as it is clothed with cilia 

 internally, and as it sends branches to the integument as well as to 

 the tentacles, it would be considered homologous as well as analo- 

 g-ous to the ambulacral system of the apodal Holothurioidea, were it 

 not observed to contain a corpusculated fluid of the same characters 

 as the blood in the perivisceral cavity. Another point of resem- 

 blance to the Echinodermata is furnished to us by the respiratory 

 cloacal trees of the Echiuridea, which in Bonellia have been very 

 clearly shown to open by numerous infundibuliform orifices into the 

 perivisceral cavity. The cloacal respiratory trees or ' lungs ' are said 

 by Semper to open similarly in the Holothurioidea ; and the resem- 

 blance between the two classes is made stronger by the fact that 

 where these respiratory trees are absent, or, as in Sijjunculus, only 

 rudimentarj^, in both classes alike we find the intestinal mesentery 

 beset with certain ciliated infundibuliform organs, which are, prob- 

 ably, connected with a water- vascular system. The muscular sys- 

 tem consists, as in Holothurioidea and Chaetophorous Annulata, 

 of an external circular and an internal longitudinal stratum, 

 to which in the Sipunculidae special retractor muscles may be 

 superadded, as in the Dendrochirotae amongst the Holothurians. 

 The chitinous armature of the proboscis, upon which these muscles 

 act in the Sipimculidae, and the peculiar prolongation of the organ 

 seen in the Echiuridea, are points which do not find any parallel in 

 the Echinodermata. The digestive tract is usually complexly con- 

 voluted ; the anus opens always on the dorsal surface, often at a 

 point in the anterior third of the body's length, as in Sipunculidae. 

 It is clothed with cilia internally, and richly glandular in its middle 

 segments, from which an oesophagus and rectum are distinguish- 

 able. The Echiuridea and Sternaspidea appear to possess a pseud- 

 haemal system, which may be distinct from the vascular system 

 already spoken of in the Sipunculidae. The nervous system con- 

 sists of a ventral cord, and of a circimi-oesophageal collar, in 

 which, however, a supra-oesophageal ganglionic mass is not always 

 present. The ventral cord, it would appear, does not ever deve- 

 lope ganglia, as it has been stated to do ; and it resembles, in 

 this absence of aggregations of nerve-cells, the band-like radial 



