1904.] NATURAL SCIENCES OF PHILADELPHIA. 389 



of the foot is derived from some of these cells". Robert describes a 

 similar condition for Trochus. In Fiona not only the median portion 

 but also much of the lateral area certainly comes from the third quartet. 

 The foot here does not arise as a paired swelling as in Patella (Patten), 

 Fulgar (Mc]\Iurrich) and Trochus (Robert), but shows from the first 

 a median protuberance which increases in size and later becomes 

 broadened and flattened (figs. 103, 108, 110). Its upper surface is 

 covered with numerous cells, but they are not arranged to form a 

 conspicuous cell-plate as in Crepidula. Large cells mark its lower 

 surface and they soon begin to secrete the operculum. 



Larval Musculature. 



It is particularly unfortunate that for a study of the muscles of the 

 velum no living material has been available, as without this many 

 points of interest must of necessity be lost. When the veliger breaks 

 from its capsule it presents an appearance shown in figs. 109, 110, 

 though it should be remembered that in fixed material, from which 

 the drawings were made, the muscles must be much contracted. The 

 whole posterior region is swollen into a huge transparent vesicle, at the 

 anterior end of which lies the contorted alimentary canal. In dotted 

 outline is represented the probable position of the cuticular-like shell 

 before shrinkage. In a larva of such age one of the most characteristic 

 features is a large dorsal retractor muscle, which has its posterior point 

 of attachment well to the left of the dorsal side of the posterior vesicle. 

 It runs forward and branches just before reaching the liver lobe, its 

 two anterior ends becoming attached to the ahmentary canal and the 

 body wall in the region of the oesophagus. In structure it is composed 

 of large spindle-shaped interlacing cells, which are flattened dorso-ven- 

 trally, giving the muscle a band-like form. In fmiction this muscle 

 doubtless acts as a retractor for the anterior and particularly the upper 

 portion of the cephalic region. A dorsal view of the same veliger 

 shows two lateral muscles, the right and left retractors of the foot, 

 which arise about midway back on the sides of the posterior vesicle 

 and extend forward through the lower part of the neck region, to end 

 in branching fibers in the foot. That of the right side is larger than 

 the left, and in earher stages (figs. 105, 106) is much thicker than later 

 and relatively larger. In figs. 105 and 106 is showoi a small muscle 

 (Vl.R.) extending from the dorsal neck region to the velar folds where 

 it branches greatly. Other similar retractor muscles of the velar 

 lobes extend from the walls of the alimentarv canal and the bodv wall 



