The Development of Phascolosoma. 11'^ 



The shedding of the yolk membrane, and the internal processes 

 of transformation which follow it, are closely connected with muscnlar 

 activity. I shall accordingly next describe the nenro-muscular 

 structures, which are formed in the trochophore as it approaches 

 the metamorphosis, and are most clearly visible during this period 

 of transition. 



In an embryo of Ph. vulgare of forty-seven hours, which has 

 shed its yolk membrane and is covered with a delicate cuticula 

 (Fig. 95), nerve fibrils have already begun to develop on the dorsal 

 or inner side of the nerve cord, which is still closely connected with 

 the ectoderm. The supraoesophageal ganglion, similarly connected, 

 may be clearly distinguished in the central and ventral part of the 

 apical region. Nerve fibrils extend from it into the oesophageal 

 connectives, which run backward around the stomodaeum and pene- 

 trate, in front, the "Punctsubstance", which constitutes the central 

 portion of its mass. A prominent apical nerve passes from the dorsal 

 part of the ganglion to the columnar 3-olk-filled cells of the rosette, 

 which still retain the apical flagella. 



A parasagittal section through the same embryo (Fig. 96) shows 

 the dorsal and ventral retractors and the neuromuscular rudiments 

 of the trunk. The fibres of the chief retractors (dorsal and ventral) 

 extend into clusters of elongated sensory cells upon the sides of the 

 introvert, in the region of the preoral circlet of cilia. It is these 

 clusters, in their earlier undifferentiated condition, that give rise to 

 the myoblasts of the retractors, as already described. The sense 

 organs of the accessory retractors are smaller groups of columnar 

 cells, which lie laterally, immediately behind the degenerating proto- 

 troch. Certain cells of the postoral band of cilia appear to have a 

 nervous connection with the accessory retractors. 



Three pairs of neuromuscular rudiments are found in the 

 trunk, viz. 



(1) two spindle-shaped clusters of elongated cells, which lie in 

 the ectoderm on each side of the ventral nerve cord, a short distance 

 behind the postoral circlet (Fig. 96 n'mu. a. v). Each of these antero- 

 ventral neuromuscular rudiments has a muscle process, which extends 

 backward through the coelom to a point near the insertion of the 

 retractors, (2) two smaller groups of cells (Fig. 96 n'mu. v. l\ which 

 lie dorsad and laterad in respect to those just described. Muscle 

 fibres extend forward from them into the zone between the proto- 

 troch and the postoral circlet. (3) A pair of single elongated cells in 



Zool. Jahrb. XXIII. Abt. f. Anat. 8 



