48 Prof. AUman on the Hydroid Zoophytes. 



lated to shake the generally received opinion of a progressive 

 organization of animals during geological time, the lower pre- 

 ceding the higher. 



But can the same be said for plants ? Coniferse are among 

 the most rife in the earliest strata in which land plants abound, 

 and these are reckoned, by some of our best botanists, the 

 highest types of plants ! Dicotyledons succeed ; and Mono- 

 cotyledons, the least complex of the flowering plants, scarcely 

 appear till Tertiary times. Has there been an inverse order of 

 creation for plants, compared with that of animals ? Is it true 

 that Palms and Bananas are inferior to the Cypress and the 

 Fir-tree ? 



V. — Notes on the Hydroid Zoophytes, By Prof. Allman. 



I. Tubularia indivisa. 



The reproductive sacs of Tubularia indivisa, though never 

 destined to become free, and belonging to the type of sporosacs 

 rather than Medusa, present nevertheless a structure in which 

 a true medusoid type may be fully recognized, and are thus of 

 especial interest in establishing the exact relation between sporo- 

 sacs and Medusae, the two forms of bodies in one or other of 

 which the generative elements of the marine Hydroid Zoo- 

 phytes always originate. 



Included within an external investment, or ectotheque, in 

 which thread-cells are imbedded, is a second sac, having a well- 

 defined opening near its summit. A circular canal, rendered 

 evident by the red pigment-granules it contains, surrounds this 

 opening. Four longitudinal canals open symmetrically into the 

 circular canal, and thence, running along the inner side of the 

 walls of the sac, enter the base of a large manubrium*, which 

 extends through the axis of the sac. 



Between the endoderm and ectoderm of the manubrium the 

 generative elements (ova or spermatozoa) are developed, and 

 when sufficiently matui'e escape, after the rupture or absorption 

 of the confining ectoderm, through the opening in the sac just 

 described, the ectotheque giving way before them, apparently by 

 rupture. 



It is impossible not to see here, in the sac which hes imme- 

 diately within the ectotheque, the umbrella of a Medusa, with 

 its orifice and its circular and radiating canals ; so that in this 

 highly interesting form of sporosac we have, with a closed 

 manubrium, all the parts amply represented which are found in 



* The diverticulum frona the coenosarc which extends through the 

 axis of the sporosac, or forms the so-called peduncle of the Medusa. 



