THE APODOUS HOLOTHURIANS 59 



tents of the tubes. In their finer structure the tubes are found to consist of 

 an outer epithelium continuous with that of the mesentery, and an inner ger- 

 minal epithelium. Between these there is always present more or less of a 

 connective-tissue layer (though this may be reduced to scattered mesenchyme 

 cells) and a more or less evident layer of longitudinal muscle fibers. In some 

 cases a layer of circular muscle fibers is also said to occur. The finer structure 

 of the genital duct is similar to that of the tubes, but the muscle layers are 

 wanting, and anteriorly the wall of the duct may even be reduced to a single 

 layer of cells. Hermaphroditism is common in the family, but many species 

 are unisexual. In the latter the germinal epithelium gives rise only to eggs 

 or spermatozoa, as the case may be, but in the former the eggs arise on one 

 side of the germinal epithelium, the spermatozoa on the other, or they arise at 

 different places on the inner side or at different times. It is possible that 

 some of the cases reported in which individuals seemed to be of different sexes 

 are simply cases of protandry; but this phenomenon is not certainly known to 

 occur among the Synaptidae, and in some cases there is no question that eggs 

 and spermatozoa are present and maturing in the same part of the genital 

 tubes and at the same time. 



EMBRYOLOGY. 



Our knowledge of the embryology of the Synaptidse is confined to a very 

 few species, particularly Labidoplax digitata, Synaptula hydriformis, Chiridota 

 rotifera, and Tceniogyrus contortus, and the following account is based upon 

 the observations recorded of them. The eggs range in size from rather more 

 than 100 microns to upwards of 330 microns. They are spherical, contain con- 

 siderable yolk, which gives them a yellowish or brownish tinge, and are more 

 or less transparent. They are generally set free in the water, but in some cases 

 develop within the body-cavity of the parent, or even in the genital tubes of 

 the mother. Nothing is known positively of the formation of the polar bodies or 

 the process of fertilization, though the latter must, in viviparous forms, take 

 place internally. Segmentation is total and approximately equal (Plate VI, 

 figs. 1 and 2) and results in the speedy formation of a blastula from which 

 the gastrjala- arises by invagination. The subsequent developmental changes 

 depend very largely upon- whether the embryos are protected within the body 

 of the mother or not. If development is so protected, it proceeds directly and 

 with considerable rapidity; but if not, the young Synaptid passes through 

 more or less prolonged larval stages. In any case the archenteron soon bends 

 to one side, and uniting with the body-wall, brings its lumen into connection 

 with the exterior, and this opening forms the water-pore and indicates the 

 dorsal surface of the animal. The archenteron continues its growth forward, 

 however, and, bending downward, unites with the ventral surface of the larva, 



