60 Dr. E. von Martens on the Occurrence of 



of fresh water to those of the sea \\ould have become smaller, 

 but the increase or decrease according to the zones would not 

 have changed^ or only unessentially. If we advance to the 

 higher steps of classification, the numerical agreement betv/een 

 the two media constantly becomes greater, but the differences 

 which still remain are of a more essential nature. This is the 

 case even in the consideration of the orders : — Of the fourteen 

 which J. Miiller has adopted for the class of Fishes, only five, 

 and these very poor in sj)ccies (with 1 — 3 genera, and not many 

 more species), are limited to one of the two media, — the Sire- 

 noidci and Ganoidei holusiei to fresh water, and the Huloccphali 

 {CJdmcera), Hyperotrcti [Mi/xine), and Lej^tocardii [Ampkioxus) 

 to the sea*. 



^ Amongst Dana^s larger sections of the Crustacea, one-half 

 (seven) in number are certainly peculiar to the sea : — Anomura, 

 Stomapoda, Schicvpoda, Ajiloopoda, Anisopoda, Merostoma, Cir- 

 ripedia, but these are all poor in species j not one is peculiar to 

 the freshwater; and of the three principal sections, Podophthalma, 

 Edriophthalma and Cirripedia, two are common. In the Anne- 

 lida, on the contrary, we find not only that the majority of the 

 orders (three to two, according to Grube) are exclusively marine, 

 but also that these are by far the most developed and most 

 numerous. In the Gasteropoda also, the exclusively marine 

 orders predominate, and hold the balance against the common 

 and freshwater orders together; thus, according to Troschel's 

 classification, there are five orders, Hetcropoda, Cyclobranchiata, 

 Notobranchiata, IMonopleurobranchiata, and Ilypobranchiata, 

 against the two common orders, Ctenobranchiata and Rhipido- 

 glossata, together with the entirely non-marine Pulmonata and 

 Pulmonata operculata (Troschel, however, excludes the Hetcro- 

 poda) ; in the more recent English systems, especially in Wood- 

 ward^s, we have the two marine orders, Nucleobranchiata and 

 Opisthobranchiata, against the common Prosobranchiata and 

 the non-marine Pulmonifera, but still of the two most numerous 

 orders, the one always includes the common, and the other the 

 non-marine forms (disregarding the Auricula?, Onchidice and Am- 

 phibola, which dwell upon the borders). An essential difference 

 for the orders, according to the zones onl}^, occurs with the 

 Fishes, the two exclusively belonging to the freshwater [Sirenoidei, 

 Ganoidei holostei), being those which are wanting in the colder 

 regions ; amongst the Gasteropoda the colder zones are destitute 

 both of the marine Heteropoda (Nucleobranchiata) and of the 

 non-marine Operculated Pulmonata, and amongst the Crustacea 



* The Berlin Museum has received an Amphioxus from Ceylou, from 

 M. Nietner. It is unfortunately not v.'ell preserved. 



