ON THE EMBRYOLOGY OF THE ECTOPROCTA. 7 



The pyriform organ lias at first sight the appearance of a 

 mucous gland, owing to the presence in it of large spaces filled 

 with a transparent substance, which does not readily take up 

 staininfif materials. A more careful examination seems, however, 

 to show that the organ is composed of a series of cells closely 

 packed together at their outer ends, and prolonged internally into 

 fine processes between which occur other cells filled with vacuole- 

 like spaces. The nuclei of the latter cells are situated, for the 

 most part, at their inner ends. It is important to notice that 

 tliere is no sharp line between the pyriform organ and the central 

 mass of nerve-fibres, which can in fact be traced into the cells 

 of the pyriform organ. It seems to me probable, from the facts 

 just described, that the principal function of the organ in question 

 is a sensory one. The larva ordinarily swims with its pyriform 

 organ directed forwards, and it is possible that this structure may 

 be of use in estimating the character of the substance on which 

 the animal desires to fix itself. The intimate connection of the 

 pyriform organ with the central nervous system, together with the 

 ciliation of the organ as a whole, is in favour of the view that the 

 structure in question is of nervous rather than of glandular nature : 

 I am unable to say whether all the cells of the pyriform organ are 

 ciliated '. 



The supposed brain of the embryo oi Alcyonidium consists then 

 of a mass of nerve-fibres partially surrounded by ganglion-cells (as 

 I identify the masses of nucleated protoplasm seen at the sides of 

 the fibrous mass in fig. 4). The ganglion-cells are connected with 

 the dorsal epiblast, except near the middle line, where a wedge- 

 shaped mass of tissue characterized by the abundance of yolk- 

 spheres intercalates itself into the nervous system. The fibrous 

 mass of the ganglion sends off a pair of strong nerves (one of which 

 is shown on the right side of fig. 4, nv.), which can be traced, in 



1 The pyriform organ has certain obvious resemblances with the structure de- 

 scribed by Kleinenberg (Zeits. f. toiss. Zool., T. xliv, 1886, p. 61) in the larva of 

 Lopadorhrjnchus as the KopfscJiild, although as the latter belongs to the prsBoral 

 region it is probably not to be regarded as the homologue of the former. The 

 Kopfschild is said to be composed of vacuolated cells, which, although not them- 

 selves of nervous nature, are in the most intimate connection with the nervous sys- 

 tem ; the organ is moreover in relation with a ciliated sense-organ. 



