ZOOLOGY AND BOTANY^ MICROSCOPY^ ETC. 247 



homologue of one of the openings of the branches of the funnel in 

 the Ctenophora. 



The third portion of the monograph deals with classification. After 

 a review of what has been done by previous writers, the author dis- 

 cusses the principles on which a system should be based, and gives a 

 phylum. The single line of descent has the Ctenophora and Coelo- 

 plana branching off, one above the other ; it then divides into two 

 branches — Acotylea and Cotylea ; of the former the three families 

 are, in ascending order, the Planoceridge, Leptoplanidte, and Cesto- 

 planidfe ; of the latter the Anonymidte, Pseudoceridse, Euryleptidfe, 

 and Prosthiostomidse ; the relative positions of the constituent genera 

 are indicated. This is followed by a short review of the tribes, 

 families, and genera, with differential diagnoses ; and this by a 

 regular account of the genera and species, in which various new 

 forms are described. A " key " is then given for the species found 

 at Naples. 



The fourth section deals with chorology and cecology; all the 

 Polyclads are marine, none being found even in brackish water. Of 

 the 226 species recognized, 66 are European and 160 exotic ; of 

 the former, 10 are incompletely known, and of the latter not one is 

 satisfactorily understood. Of the 66, 51 have been found in the 

 Mediterranean. Many Neapolitan species live under stones, or in 

 small cavities of rocks ; many between the tubes of tubicolar annelids, 

 some in mud. Mimicry and sympathetic coloration play a great 

 part in their economy, while others again are very sharply distin- 

 guished from their surroundings. Some, especially the Pseudoceridae, 

 are excellent swimmers ; many have a great capacity for regenerating 

 lost structures, and most are very retentive of life. They cannot 

 be said to be especially plentiful at any one season of the year; 

 and when one species is very abundant at a certain date, we must not 

 expect to find the same thing in the succeeding year. 



The fifth and concluding section is entitled phylogeny, and in it a 

 number of interesting problems are discussed. 



The first is that of the genetic relation of Polyclads to the Coelen- 

 terata, which was first suggested by Kowalevsky from a study of 

 Coeloplana ; limiting himself by a reflection as to the incompleteness 

 of our knowledge, the author agrees to the suggestion that Cceloplcma is 

 an intermediate form between the Triclads and the ctenophorous 

 Coelenterata ; the different organs of the two groups are carefully 

 compared in detail ; and the difficulties in the acceptance of the view 

 are next pointed out. They are the present impossibility of finding 

 anything in a Coelenterate which can be compared to the excretory 

 organs of the Turbellaria, and the apparently quite different mode of 

 development of the mesoderm in the two groups. Further investiga- 

 tions may throw some light on the cause of the differences in the 

 nervous system ; the form to be attacked just now is Cceloplana. 

 Selenka's hypothesis as to the relation of the two groups is next con- 

 sidered, and the differences between the arguments advanced by him 

 and by Lang are pointed out. 



As to the relationship of the Polyclads to the Triclads the author 



