DEVELOPMENT OF THE LANCELET. 407 



chial sacs of Ascidians and of Balanoglossus. The water 

 which enters the mouth passes out through these slits where 

 it oxygenates the blood, and enters the peribranchial cavity, 

 thence passing out of the body through the abdominal pore 

 (Fig. 387, c). The pharynx leads to the stomach (e), with 

 which is connected the liver or coscum (/). There is a 

 pulsatile vessel or tubular heart, beginning at the free end 

 of the liver, and extending along the underside of the phar- 

 ynx, sending branches to the sac and the two anterior branches 

 to the dorsal aorta. " On the dorsal side of the pharynx the 

 blood is poured by the two anterior trunks, and by the 

 branchial veins which carry away the aerated blood from 

 the branchial bars, into a great longitudinal trunk or dorsal 

 aorta, by which it is distributed throughout the body.'' 

 (Huxley.) There are also vessels distributed to the liver, 

 and returning vessels, representing the portal and hepatic 

 veins. The blood-corpuscles are white and nucleated. 



The vertebral column is represented by a notochord which 

 extends to the end of the head far in front of the nervous 

 cord ; and also by a series of small semi-cartilaginous bodies 

 above the nervous system, and which are thought to repre- 

 sent either neural spines or fin-rays. The nervous cord lies 

 over the notochord ; it is not divided into a true brain* and 

 spinal cord, but sends off a few nerves to the periphery, with 

 a nerve to the single minute eye. There are no kidneys 

 like those of the higher Vertebrates, but glandular bodies 

 which may serve as such. The reproductive glands are 

 square masses attached in a row on each side of the walls of 

 the body-cavity. The eggs may pass out of the mouth or 

 through the pore. Kowalevsky found the eggs issuing in 

 May from the mouth of the female, and fertilized by sper- 

 matic particles likewise issuing from the mouth of the male. 

 The eggs are very small, 105 millimetres in diameter. The 

 eggs undergo total segmentation, leaving a segmentation- 

 cavity. The body-cavity is next formed by invagination. 



The blastoderm now invaginates and the embryo swims 

 about as a ciliated gastrula. The body is oval, and the germ 

 does not differ much in appearance from a worm, starfish, 



* Langerhans lias figured an olfactory lobe; and all observers agree 

 that a ventricle is present ; thus there is a slight approximation to a 

 brain. 



