66 THE APODOTIS HOLOTHUBIANS 



REPRODUCTION. Reproduction in the Synaptidas is always sexual, but a 

 large number of species, probably the great majority, are hermaphroditic. 

 Fertilization ordinarily takes place in the water, outside the body of the egg- 

 producing individual, but in the viviparous forms it seems to occur in the body- 

 cavity or even in the genital ducts. So far as known, there is nothing like 

 sexual union between two individuals, but the viviparous species are often, if 

 not always, more or less gregarious, so that fertilization is thereby facilitated. 

 In the great majority of cases, the fertilized eggs undergo their development 

 in the water entirely independent of the parent, but in four species they are 

 known to be retained in the body and undergo their development there. In 

 one case (Tceniogyrus contortus) the development takes place within the gen- 

 ital tubes, which thus serve as a uterus, but in the other cases (Synaptula 

 hydriformis, Leptosynapta minuta, Chiridota rotifera) the body-cavity serves 

 as the brood-pouch. How the eggs get into the body-cavity is not certainly 

 known, but it is supposed that they pass in from the ovaries directly by rupture 

 of the covering epithelium. When and where fertilization takes place, in such 

 a case, are undetermined points, but probably it occurs after the eggs have 

 left the ovary and are free in the body-cavity. If such is the case, the sper- 

 matozoa must pass into the ccelum either directly from the hermaphroditic 

 gonads or else indirectly through the wall of the -rectum, openings in which 

 are present in Synaptula hydriformis. In Tceniogyrus contortus the sexes 

 are separate and the spermatozoa must enter the uterus from the outside water. 

 The young remain in the body-cavity of S. hydriformis until 5 mm. or more 

 (sometimes 20 mm.) in length, and are then born by a rupture of the body- 

 wall near the rear of the body. In T. contortus the young remain in the uterus 

 until 3 mm. long, and then pass out through the genital opening. Little is 

 known as to the frequency with which young are produced or the number of 

 eggs matured at each breeding period, but some of the tropical Synaptas seem 

 to breed throughout the year, while the more northern species are known to 

 mature their reproductive cells only in the spring or summer. The number of 

 eggs produced at each sexual period seems to range from five to several hun- 

 dred, but the exact limits are not known in even one species. 



ECOLOGY. 



The Synaptidae are all marine animals and have a bathymetrical range 

 from above low-water mark to at least 4,200 meters. The very great majority, 

 however, are littoral forms and there are few species occurring at depths of over 

 350 meters. They are found in all parts of the world, but seem to be most 

 abundant in the region southeast of Asia, where more than half the reported 

 species are said to occur. They appear to be least abundant in the region west 

 of South America, where no species is known to occur. An unusual number 



