On the Structure o/'Magelona. 149 



tine ; while to the junction of the first and second is attached the 

 proboscis. The pharyngeal division is furnished with complex 

 muscular layers and con\ oluted internal surface, and it is thrust 

 into the base of the proboscis in full expidsion. It is probably 

 the homologue of the proventriculus in such Annelids as the 

 Syllidae. The proboscis, again, is an instance of the separation and 

 modification of a part of the digestive canal to aid in the ceaseless 

 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 po\verful 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 pharyngeal 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 in- 

 sinuating motion) till it has advanced about its own 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 with- 

 drawal 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 retiu'uing organ, further 

 constricted by its own circular muscular coat. There is no differen- 

 tiation between the succeeding oesophageal and ventricular regions, 

 the glandular internal tunic in each being alike. The latter ceases, 

 after a marked constriction, at the beginning 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 walls 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 tw^o 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 alimentary canal, 

 receiving in each segment a large branch from the ventral trunk 

 and numerous capillaries from the intestinal wall, until the pos- 

 terior border of the tenth segment is reached. At this part their 

 dilated walls are supplied 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 



