218 



METAZOAN PHYLA 



body through the dorsal siphon and are carried by the water to another 

 individual. The egg cells do not leave the body but, owing to a reversal 

 of the direction of motion of the ciha on the walls of the suprabranchial 

 chambers, are carried into the gills and reach the marsupia. Here they 

 are fertilized by sperm cells from another mussel, which have entered 

 through the ventral siphon. A marswpium in this animal is a portion of a 

 gill modified to serve as a place for the development of the eggs. 



Larva/ 



t/7reaaf 



"Foot 



Fig. 130. — Diagram to illustrate the life history of a fresh-water mussel. A to E are 

 of TJnio complanatus (Dillwyn) ; F and G, of Lampsilis ligamentina (Lamarck). {B to E are 

 from Lillie, Journal of Morphology, vol. 10, and F and G from Lefevre and Curtis, Bxdl. Bureau 

 of Fisheries, vol. 30.) A, the adult. X about }i. B, the egg. X 55. C, Two-cell 

 stage, showing unequal cleavage. X 55. D, section of the gastrula. X 122. E, 

 glochidium. X 92. F, head of rock bass with operculum cut away to show glochidia 

 attached to the gills. G, young mussel one week after leaving the fish. Much magnified. 

 Several lines of growth have been formed. 



The eggs undergo total and unequal cleavage. Blastula and gastrula 

 stages are passed through, and a larval form known as a glochidium is 

 developed (Fig. 130). This larva has a shell consisting of two valves 

 closed by an adductor muscle. A long larval thread extends from the 

 body of the larva between the gaping valves of the shell. It is, however, 

 not present in all species. Fertilization usually occurs in a unio in spring 

 or early summer, and by the middle of August or the first part of Septem- 

 ber the glochidia have become sufficiently mature to be freed from the 



