AFFINITIES OF ECHIURUS UNICINOTUS. 93 



Sedgwick regards the Echiuroidca " as Annelids in wliidi segnioatation is teel)le, 

 showing faintly in the young, but, except in the repetition of the nephridia, .... l)eing 

 absent in the adult." 



Proboscis. — The proboscis is essentially characteristic of the Echiui'oidea, usually being 

 a long highly contractile organ, A^hich, wlien fully extended, assumes an almost ribl)ou- 

 like form. This prostomium is formed from the oral and pi-;e-oral parts of the larva. 



According to Grceff, the proboscis functions as a lung, the blood in its vessels beino- 

 separated from the sea-water only l)y a very thin layer of tissue : it thus becomes 

 oxygenated and, as he thinks that a communication exists between the blood-systeni and 

 the ccElom at the tip of the proboscis, the body-cavity fiuid in this Avay l^ecomes aerat(Hl at 

 the same time. S2)engel, as I have elsewhere remarked, denies the existence of tiiis opcTi 

 communication, and Rietsch says he has been unable to tind it. ,\.s regards the |)ro- 

 boscis sewing as a respiratory organ in Bonellia r irk/is, L., ilolando * says : " es ist kein 

 Anzeichen da, dass er ihm zum Athem oder als Kieme diene." Schmarda ao-rees with 

 Greeff in thinking that the respiratory fnnctiou is carried on by the proboscis and the 

 anal vesicles. It seems doubtful, to my mind, whetlier tlH> prol)()seis functions as a luni:, 

 if only on account of the well-known fact that it is completely thrown off on the slio-htest 

 provocation: if it subserved such an important function as respiration, it seems scarcelv 

 probable that the animal could so readily part witli it; yet in the specimens of EchlirniN 

 Pallasii which I have examined (obtained from St. Andrews) the proboscides were 

 missing, leaving in eacli case a scar round the mouth. 



Greeff also assigns to the proboscis the function of prehension of food material, 

 by definitely seizing the prey and rolling it into the mouth ; but Spengel thinks this is 

 done by the cilia on the ventral surface, apart from any action of the organ as a whole. 

 The proboscis of Echiurm uiiiciiictus differs widely from that of allied forms : it is verv 

 small, thick, and blunt, being scarcely more than a prye-oral lobe (tig. '1), and in this 

 species it is difficult to see how it can act as a lung or for the prehension of food. 

 Doubtless it is much contracted in these preserved specimens, but, even allowinii- for 

 that, it is an extremely insigniticant organ compared with that of Echiurns Fallasii, 

 Thalassema, and Bouellin. If its insignitlcance were due to great contraction, then 

 the lateral nerve-cords would be thrown into deep folds, as is tiie ventral cord ; 

 but on dissecting out the nerve-ring in the proboscis-tissue (Fl. 9. Mg. 27) it is seen to be 

 very slightly wavy, which points to the fact that the proboscis cannot be extended far 

 beyond its condition in my specimens (PI. 7. tigs. 1, 2). There is no proboscis in the 

 aberrant genus Saccosoma, described by Danielssen and Koren. 



The ventral surface is entirely clothed with cilia, which are not restricted to a groove. 



The sinuses, which are so conspicuous in the tissue of the proboscis, as seen in 

 transverse sections, I have already referred to when considering the " blood-vascular 

 system." There maj^ however, be some correlation between the develo^imeut of the 

 blood- vascular system and the proboscis. It is known that, in those larvie of tiie 

 Gephyrea whose development has been followed, the vascular system is formed late, and 



* Isis von Oben, i. 1S23, Taf. v. figs. 1 5. 

 SECOND SERIES. — ZOOLOGY, VOL. VIII. It 



