l62 



PORIFERA. 



very little, horny connecting-substance (fig. 48, d). In addition to 

 the proper skeleton-spicules, which are usually connected by a horny 

 fibre, there are developed in the mesoderm more or less numerous 

 disconnected, uniaxial, siliceous needles or " flesh-spicules." 



The order of the Monactinellidcz is represented by a very large 

 number of existing marine Sponges, as well as by the fresh-water 

 group of the Spongillidce. Owing to the nature of the hard struc- 

 tures in this order, the entire skeleton does not readily admit of pre- 

 servation, and most of the known fossil forms are therefore repre- 



Fig. 52. — Fossil Monactinellid Sponges, a, Part of the spicular network of Climacosfiongia 

 radiata, Silurian, Tennessee, enlarged twelve times ; B, Part of the spicular network of Aca.71- 

 thoraphis intertextus, Chalk, Kent, enlarged ten times ; c, Spicules of Spongilla Purbeckensis, 

 as shown in a thin slice of chert from the Purbeck formation, enlarged ; d, Spicule of Reniera 

 gracilis. Carboniferous, Yorkshire, enlarged sixty times ; E, Spicule of Reniera clavata, Carbon- 

 iferous, Yorkshire, enlarged sixty times; f, Spicule of Axinella fiaxillics, Carboniferous, Lanca- 

 shire, enlarged thirty times, (c is after John Young, and the remaining figures are after Hinde.) 



sented by mere fragments or by detached spicules. The record of 

 the distribution of the Monactinellid Sponges in past time is, there- 

 fore, necessarily a very imperfect one. The oldest types which have 

 been determined with any certainty occur in the Silurian deposits, 

 in which are found the large uniaxial spicules upon which Hinde has 

 founded the genus Atractosella. Of the same age is the genus 

 Climacospongia (fig. 52, a), in which the skeleton is composed of 

 elongated acerate spicules arranged in a ladder-like manner. Cer- 

 tain tubular borings in Silurian shells have also been referred to the 

 operations of a Sponge ( Vioa prised), supposed to be related to the 

 living boring Sponges (Cliona), but the nature of these is very doubt- 

 ful. On the other hand, fossil shells pierced by borings in all respects 

 similar to those produced by the recent Clionce are not uncommon 

 in Secondary and Tertiary deposits. In the Devonian rocks, Monac- 



