84 MICHIGAN ACADEMY OF SCIENCE. 



each new generatiou. In this stiuggle, the hirval forms of echinoderms 

 and many other invertebrates pkiy an important part, and the chances 

 of survival for any one individual are very small. Merely increasing 

 the number of ova produced therefore is not the best way of meeting 

 the difficulty. Far better will be the results of caring for' the ova and 

 young until the latter are better fitted for the struggle for life. This 

 may be done in various ways but will result almost certainly in fewer 

 eggs being produced, and these will be more vigorous and more likely 

 to produce normal, healthy young. If these young are protected by 

 the parent until well along in life, the great majority will have a chance 

 of becoming sexually mature and reproducing, and thus the species 

 will be benefited. 



We have no means of knowing how recently this change has occurred 

 among holothurians, but it is interesting to note that the great majority 

 of known cases have been discovered within about twenty years. The 

 earliest recorded instance of a holothurian caring for its young is that 

 of Si/napta vivipara discovered by Oerstedt in the West Indies in 1851. 

 In this species the young are born in the body cavity of the parent, 

 whence they escape by openings in the body-wall near the anus. The 

 eggs pass from the genital ducts directly into the body cavity, where 

 they are fertilized and develop until the young have assumed adult 

 form and are nearly an inch long. As many as 175 youug may be cared 

 for in this w^ay at one time, and their chances of survival are excellent. 

 In 18G7, Kowalevsky discovered that a Mediterranean species, Pliyl- 

 lophoriis urna, cared for its youug in the same way, and in 1881, Ludwig 

 discovered that Chiridota rot if era was also viviparous. Since 188U, no 

 less than ten sj)ecies are reported as caring for their young, but curiously 

 enough, only one of these ten carries the young in the body cavity, and 

 that one is a little tliyone [T. rubra) found on the coast of California. 

 Of the remaining nine, three brood the young under the body; in Psoitis 

 antarcticiis and Cucumaria parva, Antarctic species, and in Cuciimaria 

 ciirata of the coast of California, this is the case. No less than three 

 species, carry the young on the back; these are Cucumaria crocca and 

 Psoitis cphippifer, of the Antarctic ocean and Tit ijonc psoitis nutria »ts of 

 the California coast. Of the renuiiniug three species, (Jiiviuiiaria laevigata 

 of the Antarctic ocean and Cucumaria glacialis, an Arctic species, carry 

 the young in special brood pouches, while Chiridota contorta, an Antarctic 

 species, has the eggs develop, and the young born, in the genital ducts 

 themselves. 



These thirteen cases seem to illustrate at least two, and perhaps 

 three, perfectly' distinct lines of development. In the first place, we 

 have the line which certain synaptids are following, where the eggs 

 no longer escape into the water, but by rupture of the genital ducts 

 pass into the body cavity, or they remain in the ducts and develop 

 there. It is hard to say whether the latter is the more primitive or the 

 more specialized Avay. It is curious to note that two widely separated 

 Dendrochirota^ have also adopted this body cavity method. It seems to 

 me that this is probably the primitive arrangement and that the genital- 

 duct method of Chiridota contorta is a modifi<^d form of it. In all the 

 other cases mentioned the eggs are laid and fertilized in the water but 

 are immediately taken in charge by the mother and cared for by her. 



