PROBOSCIS COELOM. 



81 



The two dorsal canals are lined by coelomic epithelium and 

 extend back into the proboscis stalk (eh), where the left of them, 

 and sometimes the right also, opens to the exterior on the dorsal 

 side close to the junction of the stalk with the body, by the 

 proboscis pore or pores. The ventral canals also extend back 

 to the hind end of the proboscis stalk as far as the mouth, where 

 they end blindly (racemose organ) or, at most, join one another 

 owing to a deficiency in the ventral septum posteriorly (Fig. 

 66, eh v). 



The anterior end of the central complex projects into the 

 proboscis coelom to a variable extent in the different species. 



imp. 



mdv 



FIG. 64. Diagrammatic reconstruction of the anterior end of Spengelia porosa, x 10 (after 

 Punnett). The dotted line in front of the notochord indicates the forward extension of the 

 pericardial auricles. The heart is omitted, anp anterior neuropore ; en posterior cornu 

 of nuchal skeleton ; ds dorsal septum ; mdv dorso-ventral muscles (muscle-plate) ; ml 

 longitudinal muscles ; n nuchal or proboscis skeleton ; p proboscis coelom ; per peri- 

 cardium ; pnp posterior neuropore ; r dorsal root of meaullary nerve cord ; mv ventral 

 recurrent vessel ; vs ventral septum. 



In some species the notochord is continued forwards in front of 

 the other organs of the central complex for a considerable dis- 

 tance as the vermiform process (p. 75). However far it may 

 extend this anterior extension of the notochord in front of the 

 pericardium (see below) is connected to the dorsal and ventral 

 proboscis wall by a median septum consisting of dorso-ventral 

 muscle fibres which traverse the proboscis cavity and are in- 

 serted on to the notochord. These muscle fibres constitute 

 the dorso-ventral muscle plate of Spengel (Fig. 64, mdv).* An- 



'' The transverse sections figured pass behind this muscular septum, 

 excepting possibly dorsally in Fig. 65 (see footnote, p. 83). 



Z. Ill 



