DE. T. DAVIDSON ON EECENT BEACHIOPODA. 



133 



First Period. — From the Commencement of the Development of the Egg up to the 



moment toheii the Larva fixes itself 



In the least advanced state, as observed by Kowalcvsky, the egg presents the aspect 

 of a vesicle, the sides of which are composed of little cylindrical cells surrounding a 

 small cavity. Soon after a portion of a blastoderm becomes invaginated (woodcut, 

 fig. 10, A) the extremities of the gastrula approach each other, and leave between them 

 only the place of an opening. At tliis moment the embryo presents two layers of small 

 cells ; one of them is the outer layer of tlie blastodenn, the other the inner layer; and 

 it is at the expense of this last that tlie middle segment, as well as all subsequent modi- 

 fications, are formed. 



In the next stage the cavity divides itself into three lobes (one median and two lateral), 

 which communicate at the part nearest to the orifice. 

 This orifice is conuectcd with the median lobe by 

 means of a narrow canal, the walls of which present 

 two small prominences formed of small cells distinctly 

 differentiated. Soon after the embiyo divides itself 

 transversely into two segments, the one upper and 

 large (cephalic segment), the other narrower and more 

 conical (caudal segment). (Woodcut, fig. 10, B.) 



At this period tlie three lobes of tlie internal 

 cavity become separated from one another, and form 

 three isolated cavities. The median cavity presents 

 at its posterior portion so great an approximation to 

 its walls that these last appear as partitions. At the 

 lower part the median cavity is surrounded, in addi- 

 tion to its own walls, wliich are formed of cylindrical 

 cells like the envelopes of the other cavity, l)y a 

 second layer of small pale-coloured hexagonal cells, 

 which for some distance are in direct relation with 

 the external layei'. (Kowalevsky is unable to explain 

 either the origin or the signification of this laver.) 



Afterwards the median cavity is transformed into 

 a digestive tulie : perhaps these walls form the intes- 

 tino-glandular layer. The lateral cavities constitute 

 the general cavity of the body ; the interior part of 

 their walls jn-oduces the mesentery and perhaps some 

 muscular fibres (intestino-muscular layer of the 

 middle layer) ; the outer part mainly contributes to 

 the principal muscles. 



Kext follows a stage wherein the embryo is divided 

 into three segments (woodcut, fig. 10, C), the last 



Adult larva of Argiope neapolitana 

 (alter Kowalevsky). 



Swimming freely, y, eyes ; r, edge of the 

 cephalic segment ; », cylindrical part re- 

 uniting the cephalic segment to the tho- 

 racic segment ; m, mantle ; i, rudiment 

 of the iute.-tiue ; e, mesentery (a part of 

 the mantle covering over the caudal 

 segment) ; a, muscles going from the 

 dorsal pait to the ventral part of the 

 valve ; h, abdominal muscles ; c, muscles 

 of the cephalic segment ; s, bristles. 



18* 



