\1() John H. Gerould, 



become elong-ated in a direction transverse to the length of the body 

 They have at first the typical spindle shape, with a prominent oval 

 nucleus in the middle of the fibre, but even at the beginning- they 

 differ from the myoblasts of the retractors and other longitudinal 

 muscles in that they are distinctly flat. This form becomes modified 

 in the larva into that of a flat band (Fig. 98 mtt. crc). I have 

 found nothing to suggest that these muscle fibres extend as uninter- 

 rupted rings around the embryo, as Hatschek (p. 40) describes their 

 appearance in the living Sipunculus. He was unable to find the 

 nuclei of these fibres. These muscles are at first strongly developed 

 in the middle of the body immediately behind the prototroch. The 

 circular muscle fibres appear in Ph. vulgare, in which the postoral 

 circlet is borne on a row of comparatively large cells, in two bands 

 (Fig. 85), one immediately in front of the cells of the postoral circlet 

 and one directly behind them, as in Sipunculus. These myoblasts, 

 after sinking beneath the surface, become covered by the cells of the 

 postoral circlet, and constitute what may be called the middle 

 sphincter. The action of this muscle begins simultaneously with 

 that of the retractors and, as will be explained later, is of great 

 importance in the processes of metamorphosis. The circular muscles 

 do not become developed into a state of functional activity elsewhere 

 than in this special bundle of fibres, until after metamorphosis has 

 taken place. The longitudinal muscles of the body wall are still 

 later in making their appearance. 



The formation of the stomodaeura, which takes place at the 

 beginning of this period, has been described in the previous section. 

 The position of the stomodaeum is necessarily changed by the 

 growth of the trunk of the trochophore. Thus it lies at first on 

 the posterior part of the ventral surface, later in the middle of the 

 ventral surface (Fig. 49). It is always separated from the apical 

 plate by a narrow portion of the prototroch, in which respect Phascolo- 

 soma differs from Sipunculus. The prospective stomodaeal cells of the 

 latter, according to Hatschek, early sink beneath the prototroch, or 

 serosa, and are thrust forward against the apical plate. Thus in 

 the trochophore of Sipunciilus, as well as in the larva, the mouth 

 lies slightly further forward than in Phascolosoma. 



The proctodaeum is not formed until the close of this period, 

 when the trochophore has become much elongated (40 — 45 hours). A 

 narrow, cylindrical, or conical, process of endoderm grows outward 

 and dorsad from the posterior end of the solid endoderm al mass, 



