1877.] 



the Structure of Magelona. 



561 



tion and modificatiou of a part of the digestive canal to aid in the cease- 

 less perforations in the sand. Its internal surface is covered by a thick, 

 transparent chitinous layer, devoid of pores. The relaxation of its own 

 retractors, and the contraction of the muscular anterior region of the body, 

 cause it to yield readily to a powerful stream of blood sent from behind ; 

 and it smoothly unrolls from the margin of the lower lip like a very supple 

 membrane. This extrusion goes on until the brownish mass of the pha- 

 ryngeal region approaches the front of the first body-segment, when its 

 muscular coil slips into the base of the proboscis, like a plug, assisting to 

 retain the blood therein, and giving firmness to the whole organ. Thus, in 

 its progress forward, the flattened snout of the annelid is thrust amongst 

 the fine sand which it haunts (with an undulating and insinuating motion) 

 till it has advanced about its o\^ti length ; then the proboscis is ejected 

 to its full extent like an india-rubber dilator, so as to make a suitable 

 channel for the occupation of the body, while again pressing onward the 

 exploratory snout. All the retractile arrangements are next brought into 

 play ; the fan-shaped vertical muscular fibres pull in the last extruded 

 region, the short and long retractors act on the entire organ, and the 

 withdrawal of the pharyngeal protrusion makes an open channel for the 

 backward stream of blood, which rushes into the vessels of the anterior 

 region of the body out of the returning organ, further consti^icted by its 

 own circular muscular coat. There is no differentiation between the suc- 

 ceeding oesophageal and ventricular regions, the glandular internal tunic in 

 each being ahke. The latter ceases, after a marked constriction, at the be- 

 ginning of the tenth body-segment ; and thereafter the intestine, which 

 has much more lax glandular tissue and abundant fatty globules, proceeds 

 to the dorsal anus near the tip of the tail. The w^alls of this region are 

 richly furnished with capillaries ; and cilia are very evident on the 

 internal surface near the tail. 



Circulatory System. — An interesting feature is the fact that the blood 

 is a densely corpusculated fluid, the corpuscles having a pinkish colour. 

 There are two large dorsal vessels which arise, near the tip of the tail, 

 from the bifurcation of the ventral trunk. They pass forward along the 

 dorsal arch of the ahmentary canal, receiving in each segment a large 

 branch from the ventral trunk and numerous capillaries from the intestinal 

 wall, until the posterior border of the tenth segment is reached. At 

 this part their dilated walls are supphed with powerful muscles, which, 

 on the relaxation of the great muscles of the ninth segment, enable them 

 to perform the functions of contractile chambers or " hearts," and by 

 vigorous systole send the blood forward in a swift stream along the single 

 dorsal vessel of the anterior region. On arriving at the base of the snout 

 the vessel ends in the efferent branch to the tentacle on each side. The 

 current rushes along the latter (nearly at right angles to the dorsal trunk) 

 to the tips, sending off in each a web of circumferential capillaries 

 throughout the greater part of its leugth, and terminating in the afferent 



