90 ALICE L. EMBLETON ON THE STRUCTURE AND 



a partition. The inner surface of the vesicle is not ciliated, for the cilia on the spiral 

 arms do not continue in beyond the canal leading into the vesicle — in this canal the cilia 

 are directed inwards towards the interior of the vesicle. As Drasche has already pointed 

 out, there are two bands of longitudinal muscles attaching the segmental organs to the 

 body-wall. 



These nephindia have been looked upon by some authors as reproductive organs, and 

 Pallas* named them " vesiculte genitales " : this error must have arisen from the fact that 

 they function as gonodncts, though they serve only as temporary receptacles for the 

 reproductive products. They are typical nephridia ; and Rietsch says : " Les poches 

 genitales des Echiurens sont evidemmeut homologues des organes segmentaires des 

 Annelides." They have miich systematic significance ; and differences in the excretory 

 oro-ans form important distinguishing features among the various species and genera of 

 the Gephyrea. Drasche gives as one of the characteristic marks of Echiunis niiicinctus 

 " die mit Spiralrinnen versehenen Trichter der Segmentalorgane." 



Beprodiictke Organs. — In mature individuals the ova or spermatozoa are found 

 floating freely in the coelomic fluid : there are no special gonoducts, but the segmental 

 oro^ans perform the function of transmitting the ripe sexual products from the coelom to 

 the exterior. The sexes are separate in this species, though there is ajiparently no sexual 

 dimoi-phism, such as occurs, for example, so strikingly in Bonellia virkUs. 



The ova and spermatozoa are the product of the peritoneal cells lining the body-cavity, 

 in one localized area — i. e., along the ventral surface, around the ventral nerve-cord, 

 chiefly at the posterior end of the body-cavity ; they are formed by a proliferation of 

 the peritoneal cells in this region. Wiien the sexual cells are ripe they become budded 

 off into the coelom, where they float about in the perivisceral fluid until they escape to 

 the exterior through the segmental organs. In microscoiiic sections of the proboscis, 

 ova are present in the sinuses almost at the tip of that organ, showing beyond question 

 that these sinuses are merely forward extensions of the ca-lom, and are not blood-vessels. 



Greeff pictures the genital cells as budding off from a stem or rhachis ; Spengel .says 

 this axis is formed from the posterior extremity of the ventral blood-vessel : " Die Stiele 

 scheinen rundum vom einem gemeinschaftlichen Strang, einer gemeinschaftlichen 

 Rliachis, hervorzusprossen." Greeff and Spengel call this an " ovary." 



The ova are, comparatively speaking, large ; they are dark and granular, with tlie egg 

 in the centre as a clear spot or vacuole ; there is an egg-membrane or shell. 



The spermatozoa — of typical form with a globular head, and a flagellum-like tail — 

 float in the coelom in little balls or lieaps. 



Mesenteries. — Several members of the class Echiuroidea possess definite mesenteries ; 

 and many zoologists have attached great importance to these structures, claiming 

 for them a systematic significance, inasmuch as they are supposed to be indicative of 

 primitive seomentation, of which almost all trace is wanting in the adult. This is the 

 view put forward by Spengel in his observations on EcJdunis Pallasii. Referring to the 

 septum-like membrane in the region of the pharynx, he says : — " Dieses Diaphragma ist 

 eine diinne, durchsichtige, von feinen Muskelfiiden durchzogene Membran, die sich vor 



* Miscellanea Zoologiea. Hagw Comitum, 1766. 



