174 



CHAPTERS ON EVOLUTION. 



through the subsequent development and elaboration of these two 

 primary structures. Thereafter, this sponge-embryo will attach itself 

 to some fixed object ; the outside cilia, no longer required for loco- 

 motion, will disappear, and it will assume its so-called ascula form 

 (6). Other and new cilia will become developed in the inner or lining 

 membrane of the body ; the wall of the cup will next become per- 

 forated with pores ; and with the inauguration of the inward and 

 outward circulation of water, the ordinary features of adult sponge- 

 existence (7) will thus have been attained. 



Such being the course of affairs in one of the simplest of animal 

 developments, we may briefly summarise the stages included therein. 

 These stages consist firstly of the segmentation of the egg, which 

 process produces the mulberry-like mass or morula. Next in 

 order we find the planula with its two layers and its outer cilia. 

 Then succeeds the gastrula possessing an internal cavity, into 

 which a mouth shortly opens ; and with the formation of pores and 

 internal cilia, the form of the adult sponge is duly produced. 



Selecting a form of animal life widely removed from the sponges, 

 let us briefly investigate the stages through which the sea-squirts or 

 Ascidians attain the somewhat prosaic features 

 which mark their adult existence. The adult and 

 ordinary sea-squirt presents itself as a bag-shaped 

 organism (Fig. 88) rooted to stones at low water 

 mark, and bearing two apertures (Fig. 88, m, a) 

 on its upper extremity. The resemblance of these 

 ascidians to an antique wine-jar (askos) is forcible 

 enough ; and the characters from which the familiar 

 name " sea-squirt " has been derived are also readily 

 discernible. When prying humanity, even in the 

 legitimate guise of the scientific investigator, pre- 

 sumes to handle the ascidian constitution too 

 roughly, these animals are given to eject water 

 from the orifices of their jar-like bodies a playful 

 habit the unpoliteness of which, from its reflex 

 and unconscious nature, even other than scientific 

 investigators may well excuse. " Sea-squirts " are 

 usually regarded by naturalists as near relations of the oysters and 

 other molluscs ; but their differences from the familiar shellfish are 

 so numerous and so important that their separation from molluscs 

 as an aberrant type of animals and their enclosure, even in the Verte- 

 brate group, is a perfectly legitimate procedure. 



Again the aptness of the Harveian motto, "Omne vivum ex 

 ovo," is apparent, when we find that sea-squirt history begins with 

 the production and fertilisation of an egg or germ (Fig. 89, i), 

 which resembles that f the sponge and of all other animals, man 



FIG. 88. 

 SEA-SQUIRT. 



