78 



LOWER INVERTEBRATES. 



FiG.GS. — Development of JE'(«7eHr/ri«»i/ o, free-swimming plaiiula; li, about to be 

 attached; c, (/, attached; e, beginning of bydrorliiza and bydranth. 



tions of tlie colony set apart for the eajjture and digestion of food, while other portions 

 have for their only function the perpetuation of the species. It must be remembered 

 that the following account is a general one, and that there are many exceptions to it, 

 some of which will be subsequently mentioned. 



We can best understand the structure of a colony by following it bi-iefly in its 

 development. From the egg there hatches out an elongated young, known as a plan- 



iila, which freely swims 

 by means of the cilia 

 with which the sur- 

 face is covered. This 

 finally attaches itself 

 to some submerged ob- 

 ject, loses its cilia and 

 begins to develop the 

 true hydroid condition. 

 A round the upper 

 (free) end appear the 

 rudiments of the ten- 

 tacles, while the base 

 begins to divide up and 

 send out processes. 

 These latter grow and ramify in a manner strikingly like that of the roots of a tree, and 

 produce what is technically known as the hydrorhiza. From this root-like portion other 

 individuals or zooids develop, some of which are like the first, and from their greater 

 or less resemblance to .flowers, are called hydranths. These hydranths form the nutri- 

 tive portions of the colony. They may be either stalked or sessile upon the hydrorhiza. 

 Other zooids are also developed from the hydrorhiza or from the hydranth itself, 

 but these never possess the tentacles and digestive organs of the hydranths, but 

 have only reproductive functions, and are called gonangia. In these latter are devel- 

 oped small zooids which in some cases become free, in others they never separate from 

 the parent. These meduste or medusa-buds develop the male and female elements 

 (eggs and spermatozoa) which in tuni ])roduce other colonies similar to that de- 

 scribed. 



Here some very interesting questions arise, the most prominent of which is what 

 constitutes an individual ? From a single egg there is developed a number of zooids 

 from which there escape quantities of medusa^ which are frequently cajjable of feeding 

 and of reproduction. Are each of these jelly fishes, reproductive sacs, and feeding 

 portions to be regarded as separate individuals or as parts of one individual ? The 

 latter is the true course ; an individual embraces all the products of a single egg, 

 and the name zooid is applied to the various more or less independent portions, 

 which, whatever their form may be, arise by budding or fission, Ijut never by a new 

 ovarian reproduction. This distinction is somewhat different from that found in the 

 sponges. 



In a number of places in Europe and America, there has l)een found, besides ITi/dm, 

 another hydroid, living in fresh or brackish w.aters, known as Cordylophora lacustris. 

 It is a compound form, attaining a length of two inches in good specimens, and is 

 usually attached to some water-weed or to the stones in the bottom of a stream. I 

 have seen it flourishing in a stream where the current is very swift. Again it has been 



